Make sure that a circuit doesn't use relays in the same family

Moved from #28 (closed).

To do this:

• Start by implementing a "same-family?" predicate in tor-netdir, probably as a method for the Relay type.
• Then use this predicate in tor-circmgr/path/exitpath.rs.
• Implement something like the EnforceDistinctSubnets feature from C tor.