Represent Objective-C block pointers in methods as Kotlin functions

Also do some refactoring.

backend.native/tests/interop/objc/smoke.kt

@@ -8,7 +8,17 @@ fun main(args: Array<String>) {
 }
 
 fun run() {
+    println(
+            getSupplier(
+                    invoke1(42) { it * 2 }
+            )!!()
+    )
+
     val foo = Foo()
+
+    val classGetter = getClassGetter(foo)
+    invoke2 { println(classGetter()) }
+
     foo.hello()
     foo.name = "everybody"
     foo.helloWithPrinter(object : NSObject(), PrinterProtocol {