Make build_sorter work with an empty equation
Pull out Linear.hashable_equation
Allow choosing only the first N variables in min_sat

This is important because not all the variables correspond to packages any
more.
Use the sorter algorithm
Only check for one solution when bisecting
Memoize for great profit

Not really that great of profit, but we already memoize everything else, so we
might as well do it here too.
Fix wording, removing wrong comment