Represent Objective-C block pointers in methods as Kotlin functions

Also do some refactoring.

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

@@ -25,3 +25,14 @@
 @end;
 
 void replacePairElements(id <MutablePair> pair, int first, int second);
+
+int invoke1(int arg, int (^block)(int)) {
+    return block(arg);
+}
+
+void invoke2(void (^block)(void)) {
+    block();
+}
+
+int (^getSupplier(int x))(void);
+Class (^ _Nonnull getClassGetter(NSObject* obj))(void);