Dont infer string on concat

[Simn]

Currently, monomorphs are bound to String when we concat + them to a string. This PR avoids this unification and instead inserts Std.string(mono):

function main() {
	var a = null;
	trace("Value: " + a);
	$type(a); // Null<Unknown<0>> (was Null<String>)
}

This seems much more natural, given that we allow concatenating any type to a string.

Closes #11284, but probably ruins somebody's day.

[back2dos]

I like it ;)

[Simn]

This also closes #9585 and makes #9586 obsolete.