[stdlib] De-gyb and split HashedCollections.swift.gyb (NFC)

Split HashedCollections.swift.gyb into separate Set.swift and Dictionary.swift files, with some common parts going into Hashing.swift.

This is mostly a mechanical change, in preparation of unification of common parts between Set & Dictionary.

rdar://problem/34038727

Author: lorentey

stdlib/public/core/CMakeLists.txt

@@ -47,6 +47,7 @@ set(SWIFTLIB_ESSENTIAL
   CString.swift
   CTypes.swift
   DebuggerSupport.swift
+  Dictionary.swift
   DoubleWidth.swift.gyb
   DropWhile.swift
   Dump.swift
@@ -61,7 +62,6 @@ set(SWIFTLIB_ESSENTIAL
   FloatingPointParsing.swift.gyb
   FloatingPointTypes.swift.gyb
   Hashable.swift
-  HashedCollections.swift.gyb
   # WORKAROUND: This file name is not sorted alphabetically in the list because
   # if we do so, the compiler crashes.
   AnyHashable.swift
@@ -111,6 +111,7 @@ set(SWIFTLIB_ESSENTIAL
   Sequence.swift
   SequenceAlgorithms.swift
   SequenceWrapper.swift
+  Set.swift
   SetAlgebra.swift
   ShadowProtocols.swift
   Shims.swift