WIP: Add KeyedMutex

[BalmungSan]

Extention to the current Mutex class to support one look per key.

I still need to add tests. But first, I would like to receive feedback about the idea and implementation.

Some open questions I have:

1. Should we use a MapRef instead of a Ref[Map]? Could that help reduce contention?
2. Should we make the normal Mutex defer to this one, using Unit as the key? If not, should we share the LockQueueCell implementation rather than copy-paste it?

PS: Because this is a copy-paste of Mutex, IMHO it is easier to review each commit rather than the whole thing.