Partial merge from kind-backend branch, including Minisat and CNF work to

Partial merge from kind-backend branch, including Minisat and CNF work to
support incrementality.

Some clean-up work will likely follow, but the CNF/Minisat stuff should be
left pretty much untouched.

Expected performance change negligible; slightly better on memory:
  http://church.cims.nyu.edu/regress-results/compare_jobs.php?job_id=3705&reference_id=3697&mode=&category=&p=5
Note that there are crashes, but that these are exhibited in the nightly
regression run too!