Add MDynamic to late-bind monomorphs to Dynamic

[Simn]

The typer generally avoids binding monomorphs to Dynamic, which has been causing several issues. With this change, unification adds a MDynamic flag to the monomorph and then infers Dynamic if there has been no other unification by the time the monomorph is closed.

This means that it's still a monomorph during typing, as we can see here:

function getArrayDynamic():Array<Dynamic> {
	return ["foo"];
}

function main() {
	var a = getArrayDynamic();
	var b = a[0];
	$type(b); // Unknown<0>
	trace(b);
}

However, the dump now shows [Var b<0>(VUsedByTyper VAnalyzed):Dynamic], which confirms that the monomorph has been bound. This should always be preferable to having dangling open monomorphs, especially when they could appear in a function signature and could be inferred from the outside.

This shouldn't break anything but there's always a chance some code somehow relied on this behavior. It would be good to do a brief check @yuxiaomao.

[yuxiaomao]

Seems good for Shiro's projects.