Optimize iteration over CharSequence's on JVM

KT-7931 Optimize iteration over strings/charsequences on JVM

compiler/testData/codegen/box/controlStructures/forInCharSequence.kt

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    var s = ""
+    for (c in StringBuilder("OK")) {
+        s += c
+    }
+    return s
+}

compiler/testData/codegen/box/controlStructures/forInCharSequenceMut.kt

@@ -0,0 +1,19 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    var clist = listOf('1', '2', '3', '4')
+    var res1 = ""
+    for (ch in clist) {
+        res1 += ch
+        clist = listOf()
+    }
+
+    var s = "1234"
+    var res2 = ""
+    for (ch in s) {
+        res2 += ch
+        s = ""
+    }
+
+    return if (res1 == res2) "OK" else "'$res1' != '$res2'"
+}

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