[stdlib] Declare conformance of Random-Access Collections

Some of these collections were still only declared to conform to
Collection.

Author: Dave Abrahams

stdlib/public/core/Character.swift

@@ -136,7 +136,7 @@ public struct Character :
     return UInt64(Builtin.zext_Int63_Int64(value)) | (1<<63)
   }
 
-  internal struct _SmallUTF8 : Collection {
+  internal struct _SmallUTF8 : RandomAccessCollection {
     init(_ u8: UInt64) {
       let utf8Count = Character._smallSize(u8)
       _sanityCheck(utf8Count <= 8, "Character with more than 8 UTF-8 code units")
@@ -198,7 +198,7 @@ public struct Character :
     var data: UInt64
   }
 
-  struct _SmallUTF16 : Collection {
+  struct _SmallUTF16 : RandomAccessCollection {
     init(_ u8: UInt64) {
       let count = UTF16.transcodedLength(
         of: _SmallUTF8(u8).makeIterator(),

stdlib/public/core/CocoaArray.swift

@@ -25,7 +25,7 @@ import SwiftShims
 /// `Collection` conformance.  Why not make
 /// `_NSArrayCore` conform directly?  It's a class, and I
 /// don't want to pay for the dynamic dispatch overhead.
-internal struct _CocoaArrayWrapper : Collection {
+internal struct _CocoaArrayWrapper : RandomAccessCollection {
   var startIndex: Int {
     return 0
   }

stdlib/public/core/Mirror.swift

@@ -525,7 +525,7 @@ internal extension Mirror {
   /// appropriate for random access!  To avoid this pitfall, convert
   /// mirrors to use the new style, which only present forward
   /// traversal in general.
-  internal struct LegacyChildren : Collection {
+  internal struct LegacyChildren : RandomAccessCollection {
     init(_ oldMirror: _Mirror) {
       self._oldMirror = oldMirror
     }