Move Context::stop_all to Address

[thomaseizinger]

A Context is usually not available other than from within an actor's Handler and thus, this feature cannot be used from all places.

I'd claim that most likely, a piece of code shutting down all instances of a set of actors will be outside those actors and thus, would likely want to do this from the Address. An Address can still be acquired from a Context so this can still be used from within a Handler.

Additionally, to make progress on #91, we need to remove as many APIs from Context as possible and this one is using the sender which is already embedded in an Address so it is easy to port.

[thomaseizinger]

We agreed that this fits better onto Context! See linked PRs for discussion :)