Extend Map::set to return the value set instead of nothing.

Improves code generation when inlined for @:arrayAccess setter, and besides which it always feels strange having a setter that does not return a value, especcialy when haxe property setters return values.

Whilst this is a change to the std API, it should not be a breaking change at all (As opposed to if we did the opposite, removing a return from a function).