[String] Custom iterator for UTF16View (#20929)

Defining a custom iterator for the UTF16View avoid some redundant
computation over the indexing model. This speeds up iteration by
around 40% on non-ASCII strings.

stdlib/public/core/StringCharacterView.swift

@@ -234,8 +234,8 @@ extension String {
 
     @inlinable
     internal init(_ guts: _StringGuts) {
-      self._guts = guts
       self._end = guts.count
+      self._guts = guts
     }
 
     @inlinable