ndstep_minimize(): Don't crash on None.update_context()
STEP._interval_difficulty(): Allow computing many intervals at once, use in ._recompute_difficulty()

This means about 25% speedup on ndstep main.  It would be 50% if we
did this *only* in _recompute_difficulty() and not also in singular
_interval_difficulty() calls as the overhead of creating numpy arrays
is huge.  But we'll fix that properly.