Re-implement MainContext channel around a manual channel

This allows us to ensure that dropping the Receiver and its GSource will
also directly drop the closure, instead of having to wait for all
Senders to disappear too.

We have to use a mutex and a shared struct for this, and as such it
makes sense to directly implement the channel as part of this shared
struct. As the std mpsc channel internally also uses mutexes this should
not cause any considerable performance difference.

At the same time also simplify some more code and add a few more tests.

Fixes #454