Reject abstract operator overloads if nullability doesn't match

[Simn]

The idea here is to resolve abstract operator overloads only if the nullability of the expression type matches the nullability of the argument.

Not ready yet, I just need an issue number for tests.

[Simn]

Hmm, this was way easier than expected... one change I'm making here is to remove @:notNull from Int64 on the flash target, because I don't think that's correct. The native representation on that target is a class, so we shouldn't pretend otherwise.

@yuxiaomao Could you check some of your codebases with this PR?

[Simn]

As expected there are some problems for cases such as this:

abstract T(Float) from Float {
	@:op(A + B) public function addFloat(f:Float) {
		return this + f;
	}
}

function main() {
	var t:T = 0.0;
	var nullFloat:Null<Float> = 1.0;
	trace(t + nullFloat); // T should be Float
}

The binop used to resolve to the @:op function, but with Null<Float> and Float having different nullability, that no longer works. I consider this an improvement, but it remains to be seen if this is something we can adapt to.