Merge: Allow keys to be nullable in `Map` and `MapIterator`

I need at least the first commit since MapIterators can be used with non-maps. The second commit is more for consistency. Note that HashMap keys are still non-null.

If this is of interest to you, I'll complete and publish my `NullableMap` in `more_collection`.

Pull-Request: #1041
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>