Add an immutable.StrictOptimizedSortedSetOps

This will be mixed into TreeSet and BitSet and has two purposes:

- Make TreeSet and BitSet extend `collection.StrictOptimizedIterableOps`
  with `CC = immutable.Set` instead of `CC = collection.Set` as before.
  This is important since the latter is incoherent and potentially
  unsound. Scala 2 does not detect this (scala/bug#11042), but Dotty does
  and emit an error.

- Make TreeSet and BitSet use more optimized versions of some methods.

src/library/scala/collection/immutable/SortedSet.scala

@@ -38,6 +38,11 @@ trait SortedSetOps[A, +CC[X] <: SortedSet[X], +C <: SortedSetOps[A, CC, C]]
   def unsorted: Set[A]
 }
 
+trait StrictOptimizedSortedSetOps[A, +CC[X] <: SortedSet[X], +C <: SortedSetOps[A, CC, C]]
+  extends SortedSetOps[A, CC, C]
+    with collection.StrictOptimizedSortedSetOps[A, CC, C]
+    with StrictOptimizedSetOps[A, Set, C]
+
 /**
   * $factoryInfo
   * @define coll immutable sorted set