[String.Index] Simplify and prepare for more resilience.

Simplify String.Index by sinking transcoded offsets into the .utf8
variant. This is in preparation for a more resilient index type
capable of supporting existential string indices.

stdlib/public/core/StringRangeReplaceableCollection.swift

@@ -455,11 +455,8 @@ extension String {
 
   @inlinable // FIXME(sil-serialize-all)
   internal func _stride(of i: Index) -> Int {
-    if case .character(let stride) = i._cache {
-      // TODO: should _fastPath the case somehow
-      _sanityCheck(stride > 0)
-      return Int(stride)
-    }
+    if let stride = i.characterStride { return stride }
+
     let offset = i.encodedOffset
     return _visitGuts(_guts, args: offset,
       ascii: { ascii, offset in