Handle followup tickets from stream isolation

As a followup to @trinity-1686a's work on stream isolation, we should do a pass to polish things up, think about names we want, and finish our remaining arguments ^W discussions.

• Write a ticket summarizing the discussions we had about which traits to expose and how. ← This is #416
• Revise and extend the documentation for Isolation, IsolationHelper, and all related functions.
• Make sure test coverage is very high.
• Write even more example code.
• Consider renaming all "isolation_group" functions in tor-client to just talk about "isolation`.
• Consider alternatives to the current set_isolation_group signature.
• Consider removing the StreamIsolation object as a separate type, or refactoring how it is used. \
  e.g., if we did impl<A,B> IsolationHelper for (A,B) where A:IsolationHelper, B:IsolationHelper then we could just have CircMgr be written in terms of an Isolation.
• Investigate whether we can/should have a design that supports for faster-than-linear circuit lookup.
• Decide whether we want to seal the Isolation trait.