Automatically emit objc_msgSend calls when calling imported or implemented Objective-C methods

- Add intrinsics.objc_super()

- Emit objc_msgSendSuper2 calls when an objc method call is combined with objc_super(self)

- Fix objc_block return value ABI for large struct returns

- Fix objc_implement method wrappers bad ABI for large struct returns and indirect args

- Simplify parameter forwarding for objc_imlpement methods

- Add intrinsics.objc_instancetype to mimi Objective-C instancetype* returns
  This facilitates returning the correct type on subclasses when calling mehtods
  such as `alloc`, `init`, `retain`, etc.

- Refactor Objective-C class implementations generation so that hierarchies are properly initialized

- Better codegen for context passing with ivar-based autocontext

- Allow @superclass on imported objc-c objects

- Better codegen for block forwarding invoker, arguments are forwarded directly

src/checker.cpp

@@ -1416,6 +1416,8 @@ gb_internal void init_universal(void) {
 		t_objc_SEL      = alloc_type_pointer(t_objc_selector);
 		t_objc_Class    = alloc_type_pointer(t_objc_class);
 		t_objc_Ivar     = alloc_type_pointer(t_objc_ivar);
+
+		t_objc_instancetype = add_global_type_name(intrinsics_pkg->scope, str_lit("objc_instancetype"), t_objc_id);
 	}
 }
 
@@ -3386,12 +3388,18 @@ gb_internal void init_core_map_type(Checker *c) {
 	t_raw_map_ptr       = alloc_type_pointer(t_raw_map);
 }
 
+gb_internal void init_core_objc_c(Checker *c) {
+	t_objc_super     = find_core_type(c, str_lit("objc_super"));
+	t_objc_super_ptr = alloc_type_pointer(t_objc_super);
+}
+
 gb_internal void init_preload(Checker *c) {
 	init_core_type_info(c);
 	init_mem_allocator(c);
 	init_core_context(c);
 	init_core_source_code_location(c);
 	init_core_map_type(c);
+	init_core_objc_c(c);
 }
 
 gb_internal ExactValue check_decl_attribute_value(CheckerContext *c, Ast *value) {