action-set: a jujuc command for Actions to send values back to state

action-set permits action hooks to define an arbitrary map of params
which will be returned to the state server at the end of the Action,
or to specify --failed with an optional failure message.
wait, what? is this just calling a copy of all the logic in uniter/Context? this doesn't really aid the simplicity and clarity of the tests...

Instead of storing an ActionResults and mutating it, why not just record the calls that were made and check they match what's expected? It makes the tests pretty much trivial, but that's kinda the point: we can fully test ActionSet by just checking what it does to a mocked-out Context when passed particular args, and we can trust the implementation of Context itself (assuming UpdateActionResults is properly tested there, ofc) to DTRT in a more functional context (eg uniter_test.go -- in which it'd probably be good form to add at least one tracer-bullet test for action-set when you can).