optimize reordering of ml-matches to avoid unnecessary computations

This now chooses the optimal SCC set based on the size of lim, which
ensures we can assume this algorithm is now << O(n^2) in all reasonable
cases, even though the algorithm we are using is O(n + e), where e may
require up to n^2 work to compute in the worst case, but should require
only about n*min(lim, log(n)) work in the expected average case.

This also further pre-optimizes quick work (checking for existing
coverage) and delays unnecessary work (computing for *ambig return).