Supervisor is underpowered

[SystemFw]

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I feel like some of the internal machinery inside Supervisor would be quite useful to expose in its external interface too. The current api has the advantage of mimicking the start api, but it means that Supervisor is useful in fewer cases than it could. In particular, I think it would be useful to be able to cancel a specific Fiber from Supervisor, but currently to do that you have to rebuild the exact same machinery, i.e. a Map[Unique, Fiber], in user code. Thoughts?

[djspiewak]

Sounds exactly like this: #1345 :-)