Cache.v: checkpoint some proof steps
Cache.v: checkpoint some proof steps

need to fix proofs to take into account the cached number of elements
in the buffer cache.  annoyingly tricky.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl