Need better up/down-enabled rooms and better management of these in the generator. Currently the generator has too small a chance of matching up/down exits.

Cutting this from 0.0.4. Exact matching requires 2^14 rooms in the library. Not sure what to do about that, as dead-end doorways will be strange.

This requires either prioritizing up/down over other exit matching, OR forcing perfect matches only (which will require a larger, permutation-complete room library).

It may be more efficient to use a bitmask for doorways and compare to that for exact matches.