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.
I see what you mean.  It seems to me like the map interleaving doesn't belong as a Context method.  The fact that you can't return map copies is a big pet peeve of mine with Go, but the intuitive response then is that I'm doing something a way it shouldn't be done; on the other hand, I like the way I'm doing the map interleave in action-set... so I'm open to suggestions.