Code review comment for lp:~robert-ancell/mir/frontend-class-renaming

i'm going to mark as 'needs information' but what I really mean is 'needs discussion'. I guess if the binder code that drove these abstractions is now gone, it would make sense for the abstractions to be reworked. The only part that I don't like is that we're getting rid of MessageProcessor's interface class, will make it harder to mock.

I guess overall I agree with the reasons and the substance for the changes, but don't see the test scenario that's driving the changes that go beyond the name changes (eg, making process_message public instead of private)