is_valid method in Component trait

[jonahweissman]

We can separate some concerns and nudge component authors to think more about their state space by requiring a is_valid(State) -> bool method and performing this check before passing any state change requests to components. (This check would normally be performed by components in their changeState implementation.) By breaking this logic out, we can make our "message is well formatted and is about to be acted upon" confirmation reply more meaningful (right now, components can still complain that the message is poorly formatted after the controller has replied with a confirmation).

[dmeliza]

I think this is linked to #10 and can be probably be closed. Will wait for confirmation from @bpqle.

[bpqle]

The check for validity seems unnecessary, I think.