J2N.Collections: Added AsReadOnly() method to each dictionary and set

[NightOwl888]

Closes #93.

J2N.Collections: Added AsReadOnly() method to each dictionary and set to ensure it is unaffected by Micorsoft's extension methods of the same name in net core. This ensures read-only collections of the built-in types will always respect structural equality and structural formatting.

There will still be collisions using .AsReadOnly() if declaring using J2N.Collections.Generic.Extensions; and using System.Collections.Generic;. In those cases, the user must call the static method that they prefer explicitly.

Our extension method existed prior to Microsoft's. We followed a best practice of putting it into a separate namespace than J2N.Collections.Generic so users would have an option to not import the J2N.Collections.Generic.Extensions if there were a conflict with another library. It was assumed that Microsoft would declare AsReadOnly() on each collection (as it is on List<T>) rather than using an extension method so there would be no conflict. Or at least that they would follow the best practice of not including it directly in the System.Collections.Generic namespace. Unfortunately, they did neither of those things.

New APIs:

namespace J2N.Collections.Concurrent
{
    public class LurchTable<TKey, TValue>
    {
        public ReadOnlyDictionary<TKey, TValue> AsReadOnly(); 
    }
}

namespace J2N.Collections.Generic
{
    public class Dictionary<TKey, TValue>
    {
        public ReadOnlyDictionary<TKey, TValue> AsReadOnly();
    }
    public class HashSet<T>
    {
        public ReadOnlySet<T> AsReadOnly();
    }
    public class LinkedDictionary<TKey, TValue>
    {
        public ReadOnlyDictionary<TKey, TValue> AsReadOnly();
    }
    public class LinkedHashSet<T>
    {
        public ReadOnlySet<T> AsReadOnly();
    }
    public class SortedDictionary<TKey, TValue>
    {
        public ReadOnlyDictionary<TKey, TValue> AsReadOnly();
    }
    public class SortedSet<T>
    {
        public ReadOnlySet<T> AsReadOnly();
    }
}