Optimize for-in-string loops

For-in-string loop can be generated using specialized 'length' and
'charAt' method calls, and with cached string length.
Note that update of the string variable in loop body doesn't affect
loop execution semantics.

 #KT-21322 Fixed Target versions 1.2.20

compiler/testData/codegen/bytecodeText/forLoop/forInStringSpecialized.kt

@@ -0,0 +1,15 @@
+// WITH_RUNTIME
+
+fun test() {
+    var s = ""
+    for (c in "testString") {
+        s += c
+    }
+}
+
+// 0 iterator
+// 0 hasNext
+// 0 nextChar
+// 0 INVOKEINTERFACE
+// 1 ISTORE 4
+// 1 ILOAD 4