Simplify circuits

[jcorbin]

• drop self pattern
• make Circuit better encapsulated by injecting the root
• drop another layer of event-emission coupling
• drop unused locked states

r @raynos @rf @kriskowal

[kriskowal]

:ok: This looks correct. It might be worthwhile to flatten the state objects themselves onto the circuit. We probably mis-predicted a need for extensibility here.

[kriskowal]

:ok: up to this point now