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);

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 {

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

@@ -47,3 +47,13 @@ void replacePairElements(id <MutablePair> pair, int first, int second) {
     [pair update:0 add:(first - pair.first)];
     [pair update:1 sub:(pair.second - second)];
 }
+
+int (^getSupplier(int x))(void) {
+    return ^{
+        return x;
+    };
+}
+
+Class (^ _Nonnull getClassGetter(NSObject* obj))() {
+    return ^{ return obj.class; };
+}