* bhc: cache pairwise log Bayes factors between merges

bhc_merge scored every active pair afresh at every merge, K³·M lgamma
calls over a run. The pair scores are now kept between merges: a merge
drops the two children's pairs and scores only the new node against the
rest, in parallel, so a run costs K²·M. The active list stays in arena
order, so every pair is scored with the same argument order as before
and the merges and their log BFs come out bit for bit the same; a unit
test checks this against the old loop, ties and empty clusters included.

* version 0.8.2