Merge pull request #430 from natecook1000/nc-scalarindex-nocore

Update for new string index inits

Author: gribozavr

Foundation/String.swift

@@ -74,7 +74,10 @@ 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