Unclosed monomorphs in function signature

[Simn]

I randomly found this problem today in the test for #2685:

function main() {
	var a:Int = func1(1);
	$type(func1); // (a : Dynamic) -> Int
	var s:String = func1("foo"); // Int should be String
}

function func1(a:Dynamic) {
	return a;
}

The problem is that the return type of func1 remains a monomorph because those are not bound to Dynamic. It is instead unified with the type of var a:Int and thus the function itself ends up as (a : Dynamic) -> Int, which is just wrong.

Very related to #3033 and I still maintain that functions should not expose their monomorphs. I'll try to see what happens if we bind leftover monomorphs to Dynamic (and maybe Void for some return types?), but I expect some friction here.