SQUASH ME: Unfortunate @_specialize addition.

_appendUTF16 is perhaps too general, and only getting partially
specialized based on CodeUnit. This compounds with
Character._SmallUTF16 not getting specialized produces a ton of silly
witness table dispatches.

This shaves off around 30% from StringEdits.

Author: milseman

stdlib/public/core/StringStorage.swift

@@ -222,6 +222,7 @@ extension _SwiftStringStorage {
 
   @_inlineable
   @_versioned
+  @_specialize(where C == Character._SmallUTF16, CodeUnit == UInt8)
   @nonobjc
   internal final func _appendInPlaceUTF16<C: Collection>(contentsOf other: C)
   where C.Element == UInt16 {