Unregister `after` timeouts

[NicolasT]

With #36, registerDelay is used for every receive clause with an after match. This registers a timeout in the RTS. Now, when the timeout is not reached (i.e., a matching message is received before the after clause fires), this leaks such timeout.

This could be an issue in practice, especially when "long" timeouts are used.

There's no way to unregister timeouts when using the registerDelay API (since it doesn't return any TimeoutKey), but it's likely possible to implement this, basically duplicating the implementation of registerDelay from base, now returning the TimeoutKey which can then be unregisterTimeout'ed once dequeue returned (ideally using some finally construct, of course).