[String] Refactor helper code into UnicodeHelpers.swift.

Clean up some of the index assumptions, stick index-aware methods on
_StringGuts, and otherwise migrate code over to UnicodeHelpers.swift.

stdlib/public/core/StringGuts.swift

@@ -341,3 +341,17 @@ extension _StringGuts {
   }
 }
 
+// Index
+extension _StringGuts {
+  @usableFromInline
+  internal typealias Index = String.Index
+
+  @inlinable
+  internal var startIndex: String.Index {
+    @inline(__always) get { return Index(encodedOffset: 0) }
+  }
+  @inlinable
+  internal var endIndex: String.Index {
+    @inline(__always) get { return Index(encodedOffset: self.count) }
+  }
+}