[stdlib] Remove _StringCore reference from UnicodeScalarIndex

This removes the `_core` property from UnicodeScalarView.Index
and moves any remaining index-moving logic from the index to
the view in UnicodeScalarView and CharacterView.

stdlib/public/SDK/Foundation/NSStringAPI.swift

@@ -96,7 +96,10 @@ extension String {
   /// Return an `Index` corresponding to the given offset in our UTF-16
   /// representation.
   func _index(_ utf16Index: Int) -> Index {
-    return Index(_base: String.UnicodeScalarView.Index(utf16Index, _core))
+    return Index(
+      _base: String.UnicodeScalarView.Index(_position: utf16Index),
+      in: characters
+    )
   }
 
   /// Return a `Range<Index>` corresponding to the given `NSRange` of