For-loop generation rewritten for the case of explicit iterator usage

This implementation includes multi-declarations already, but tests are coming in the next commits.
Old ForLoopgenerator renamed to IntrinsicForLoopGenerator, and will be unified with the newly introduced interface soon.

Side-effect:
KT-2668 Codegen doesn't handle member extensions in for
 #KT-2668 Fixed

compiler/testData/codegen/controlStructures/forLoopMemberExtensionHasNext.kt

@@ -0,0 +1,23 @@
+class It {
+    fun next() = 5
+}
+
+class C {
+    fun iterator(): It = It()
+}
+
+class X {
+    var hasNext = true
+    fun It.hasNext() = if (hasNext) {hasNext = false; true} else false
+
+    fun test() {
+        for (i in C()) {
+            System.out.println(i)
+        }
+    }
+
+}
+
+fun main(args: Array<String>) {
+    X().test()
+}