Fix a variety of issues in FrozenCollections

- Add the new ToFrozenDictionary/Set methods that take optimizedForReading and make the existing overloads cheaper for construction.  This makes construction time and allocation by default much closer to that of a normal dictionary or set while also enabling more of the implementation to be trimmed away by default.
- Fixed the use of FrozenSet/Dictionary.Empty such that we only use the singleton when its comparer matches; otherwise, we could end up handing back an instance with the wrong comparer.
- Avoided passing both a dictionary/set and a comparer down through constructors, as the dictionary/set contains the comparer and we don't want to accidentally end up in a situation where we pass through the wrong one.
- Condensed some of the construction code to be a bit less verbose
- Removed unused locals from ToFrozenDictionary (the min/max/range were no longer being used)
- Remove unnecessary array allocations from PickIntegerDictionary
- Delete the sparse range integer sets, which are rarely needed and are broken for TryGetValue
- Fix CheckUniqueAndUnfoundElements to slice down to only the relevant size prior to clearing

src/libraries/System.Collections.Immutable/ref/System.Collections.Immutable.cs

@@ -9,6 +9,7 @@ namespace System.Collections.Frozen
     public static partial class FrozenDictionary
     {
         public static System.Collections.Frozen.FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(this System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> source, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) where TKey : notnull { throw null; }
+        public static System.Collections.Frozen.FrozenDictionary<TKey, TValue> ToFrozenDictionary<TKey, TValue>(this System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> source, System.Collections.Generic.IEqualityComparer<TKey>? comparer, bool optimizeForReading) where TKey : notnull { throw null; }
         public static System.Collections.Frozen.FrozenDictionary<TKey, TSource> ToFrozenDictionary<TSource, TKey>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, TKey> keySelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) where TKey : notnull { throw null; }
         public static System.Collections.Frozen.FrozenDictionary<TKey, TElement> ToFrozenDictionary<TSource, TKey, TElement>(this System.Collections.Generic.IEnumerable<TSource> source, System.Func<TSource, TKey> keySelector, System.Func<TSource, TElement> elementSelector, System.Collections.Generic.IEqualityComparer<TKey>? comparer = null) where TKey : notnull { throw null; }
     }
@@ -71,6 +72,7 @@ public partial struct Enumerator : System.Collections.Generic.IEnumerator<System
     public static partial class FrozenSet
     {
         public static System.Collections.Frozen.FrozenSet<T> ToFrozenSet<T>(this System.Collections.Generic.IEnumerable<T> source, System.Collections.Generic.IEqualityComparer<T>? comparer = null) { throw null; }
+        public static System.Collections.Frozen.FrozenSet<T> ToFrozenSet<T>(this System.Collections.Generic.IEnumerable<T> source, System.Collections.Generic.IEqualityComparer<T>? comparer, bool optimizeForReading) { throw null; }
     }
     public abstract partial class FrozenSet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable
     {

src/libraries/System.Collections.Immutable/src/System.Collections.Immutable.csproj

@@ -10,6 +10,7 @@ The System.Collections.Immutable library is built-in as part of the shared frame
 
   <ItemGroup>
     <Compile Include="Properties\InternalsVisibleTo.cs" />
+
     <Compile Include="System\Collections\Frozen\String\KeyAnalyzer.cs" />
     <Compile Include="System\Collections\Frozen\String\OrdinalStringFrozenDictionary_FullCaseInsensitiveAscii.cs" />
     <Compile Include="System\Collections\Frozen\String\OrdinalStringFrozenDictionary_FullCaseInsensitive.cs" />
@@ -142,18 +143,18 @@ The System.Collections.Immutable library is built-in as part of the shared frame
     <None Include="Interfaces.cd" />
   </ItemGroup>
 
+  <ItemGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">
+    <Compile Include="System\Collections\Frozen\WrappedDictionaryFrozenDictionary.cs" />
+  </ItemGroup>
+
   <ItemGroup Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))">
-    <Compile Include="System\Collections\Frozen\Integer\IntegerFrozenDictionary.cs" />
-    <Compile Include="System\Collections\Frozen\Integer\IntegerFrozenSet.cs" />
     <Compile Include="System\Collections\Frozen\Integer\SmallIntegerFrozenDictionary.cs" />
     <Compile Include="System\Collections\Frozen\Integer\SmallIntegerFrozenSet.cs" />
-    <Compile Include="System\Collections\Frozen\Integer\SparseRangeIntegerFrozenSet.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))">
     <Compile Include="System\Collections\Frozen\Int32\SmallInt32FrozenDictionary.cs" />
     <Compile Include="System\Collections\Frozen\Int32\SmallInt32FrozenSet.cs" />
-    <Compile Include="System\Collections\Frozen\Int32\SparseRangeInt32FrozenSet.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'">

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/DefaultFrozenDictionary.cs

@@ -12,8 +12,8 @@ namespace System.Collections.Frozen
     internal sealed class DefaultFrozenDictionary<TKey, TValue> : KeysAndValuesFrozenDictionary<TKey, TValue>, IDictionary<TKey, TValue>
         where TKey : notnull
     {
-        internal DefaultFrozenDictionary(Dictionary<TKey, TValue> source, IEqualityComparer<TKey> comparer)
-            : base(source, comparer)
+        internal DefaultFrozenDictionary(Dictionary<TKey, TValue> source, bool optimizeForReading)
+            : base(source, optimizeForReading)
         {
         }
 

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/DefaultFrozenSet.cs

@@ -9,8 +9,8 @@ namespace System.Collections.Frozen
     /// <typeparam name="T">The type of the values in the set.</typeparam>
     internal sealed class DefaultFrozenSet<T> : ItemsFrozenSet<T, DefaultFrozenSet<T>.GSW>
     {
-        internal DefaultFrozenSet(HashSet<T> source, IEqualityComparer<T> comparer)
-            : base(source, comparer)
+        internal DefaultFrozenSet(HashSet<T> source, bool optimizeForReading)
+            : base(source, optimizeForReading)
         {
         }
 

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/EmptyFrozenDictionary.cs

@@ -11,7 +11,7 @@ namespace System.Collections.Frozen
     internal sealed class EmptyFrozenDictionary<TKey, TValue> : FrozenDictionary<TKey, TValue>
         where TKey : notnull
     {
-        internal EmptyFrozenDictionary() : base(EqualityComparer<TKey>.Default) { }
+        internal EmptyFrozenDictionary(IEqualityComparer<TKey> comparer) : base(comparer) { }
 
         /// <inheritdoc />
         private protected override TKey[] KeysCore => Array.Empty<TKey>();

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/EmptyFrozenSet.cs

@@ -2,15 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Linq;
 
 namespace System.Collections.Frozen
 {
     /// <summary>Provides an empty <see cref="FrozenSet{T}"/> to use when there are zero values to be stored.</summary>
     internal sealed class EmptyFrozenSet<T> : FrozenSet<T>
     {
-        internal EmptyFrozenSet() : base(EqualityComparer<T>.Default) { }
+        internal EmptyFrozenSet(IEqualityComparer<T> comparer) : base(comparer) { }
 
         /// <inheritdoc />
         private protected override T[] ItemsCore => Array.Empty<T>();