Optimize iteration over CharSequence's on JVM

KT-7931 Optimize iteration over strings/charsequences on JVM

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

@@ -0,0 +1,18 @@
+// WITH_RUNTIME
+
+fun test() {
+    var s = ""
+    for (c in "testString") {
+        s += c
+    }
+    for (c in StringBuilder("testStringBuilder")) {
+        s += c
+    }
+    for (c in "testCharSequence".apply<CharSequence>{}) {
+        s += c
+    }
+}
+
+// 0 iterator
+// 0 hasNext
+// 0 nextChar