Make the Lumber instance Sync/Send

[foxfriends]

It would be nice to be able to evaluate queries in multiple threads on the same instance of Lumber. This would prevent needing to recompile the program multiple times, just to get an instance onto multiple threads. The entire unification process is single threaded so there is no reason why this should be trouble. Probably just needs the Rcs to be converted to Arcs.

Keep in mind #16 is a feature that will be added, but most likely we will be able to isolate the mutation to a separate object that is passed in, allowing the core Lumber instance to remain sync/send/usable from two threads at once.