The tests contained herein will only pass once HaxeFoundation/hxcpp#245
is accepted. It adds basic tests for the boxing and unboxing operations,
and it adds some extra code to handle `null`, and a few operators
correctly.

Also, this makes sure that `@:objc` code is only run when `-D objc` is
defined - which should make any changes safe against regressions. I've
also made sure any added code is labeled `objc` properly.

…is objc

Since all objective-c types are boxed into Dynamic as the same type,
this cast is needed in order to make sure the type is restored.
This will only kick in if `is_objc_type` is true, which only happens on
`@:objc` types.

- Use the `id <SomeInterface>` syntax for when extern @:objc interfaces
are referenced
- This commit needs the changes made by HaxeFoundation/haxe@3815c00
- No solution was yet made to check if an optional method was
implemented by the interface; On Haxe they will have to be included with
@:noCompletion and @:deprecated to satisfy the typeload