zvariant::Dict please allow iterating the key/value pairs

Hi, at the moment there is no way to just iterate the key/value pairs of a zvariant::Dict. If you want to do it you need to convert it to a hashmap, and to do that you need to read the signature and pick a concrete type that implements Basic, it's actually quite a lot of code to do this, and what I (and probably others) really want is just to iterate the values. I see that it's stored as a Vec of DictEntries, which are just the key and value, so it would be trivial to allow this iteration.

Thanks, zbus is awesome!