New tor-relay-selection crate to encapsulate relay selection choices.
This is a set of first steps towards centralizing our relay selection predicate logic in one place, so that it's hard(er) to forget all the properties that one needs to check on a relay. See #504 (closed) for more motivation.
Closes #789 (closed).
Partially completes #504 (closed).
Edited by Nick Mathewson