Controlling access, pt. 4

[UPDATED]   More currentPlayer removed.
[UPDATED]   Methods where there's a useful difference between unknown and absent values now return Maybe.
[FIXED]     Regression: movement pathfinder didn't consider remainingSpeed anymore.
[FIXED]     Don't test accessibility when leveling: units may not be able to see where they're leveling to.  Accessibility will be enforced when executing.
[FIXED]     Unit tests to comply with new API.