[stdlib][NFC] Break Set/Dictionary implementation into smaller files

This has long been overdue; splitting the Dictionary.swift/Set.swift monoliths makes for a far less confusing editing experience.

Author: lorentey

stdlib/public/core/CMakeLists.txt

@@ -52,6 +52,11 @@ set(SWIFTLIB_ESSENTIAL
   CTypes.swift
   DebuggerSupport.swift
   Dictionary.swift
+  DictionaryBridging.swift
+  DictionaryBuilder.swift
+  DictionaryCasting.swift
+  DictionaryStorage.swift
+  DictionaryVariant.swift
   DropWhile.swift
   Dump.swift
   EmptyCollection.swift
@@ -69,7 +74,6 @@ set(SWIFTLIB_ESSENTIAL
   # if we do so, the compiler crashes.
   AnyHashable.swift
   # END WORKAROUND
-  HashedCollectionsAnyHashableExtensions.swift
   Hasher.swift
   Hashing.swift
   HashTable.swift
@@ -92,6 +96,8 @@ set(SWIFTLIB_ESSENTIAL
   Mirrors.swift.gyb
   Misc.swift
   MutableCollection.swift
+  NativeDictionary.swift
+  NativeSet.swift
   NewtypeWrapper.swift
   NormalizedCodeUnitIterator.swift
   ObjectIdentifier.swift
@@ -118,6 +124,12 @@ set(SWIFTLIB_ESSENTIAL
   SequenceWrapper.swift
   Set.swift
   SetAlgebra.swift
+  SetAnyHashableExtensions.swift
+  SetBridging.swift
+  SetBuilder.swift
+  SetCasting.swift
+  SetStorage.swift
+  SetVariant.swift
   ShadowProtocols.swift
   Shims.swift
   Slice.swift