Allow interface Mixins

[Chocohead]

Reimplements #13 more expansively, allows things like...

public interface Interface {
	String GENERATED_CONSTANT = generate();

	private static String generate() {
		return "generated";
	}

	public static String make() {
		return "made";
	}

	void eat(int food);

	default int next(int start) {
		return start + 1;
	}
}

@Mixin(Interface.class)
interface InterfaceMixin {
	@Inject(method = "make", at = @At("RETURN"), cancellable = true)
	private static void remake(CallbackInfoReturnable<String> call) {
		call.setReturnValue("remade");
	}

	@ModifyVariable(method = "next", argsOnly = true, at = @At("HEAD"))
	default int switchNext(int start) {
		hit();
		return start - 1;
	}

	@Shadow
	private static String generate() {
		throw new AssertionError();
	}

	private void hit() {
		System.out.println(generate());

		Arrays.asList("one", "two", "three").removeIf(thing -> {
			//No more IncompatibleClassChangeErrors from lambdas in interfaces
			return false; //The list is fixed length so we don't actually want it to succeed
		});
	}
}

[frqnny]

Will this allow people to inject into things like Inventory and run code every time, for example, getStack or something gets called?

[sfPlayer1]

Looks reasonable to me

[Chocohead]

Will this allow people to inject into things like Inventory and run code every time, for example, getStack or something gets called?

No, this is strictly for Mixining into the interface itself. With Inventory that means you could inject into onOpen or onClose and anything using the default implementation would then run your injection, for example, but like with abstract classes you can't inject into an abstract method as it has no method body to inject into.

[LambdAurora]

Quite an essential PR that I hope to see merged soon.

[Mysterious-Dev]

LGTM