model: add release sequence support

model: add release sequence support

The ModelChecker now can find the head(s) of the release sequence(s) with which
a particular ModelAction (read-acquire) will synchronize.

The ModelChecker::release_seq_head function can locate a release sequence head
for a given ModelAction, based on information at a given moment. That is, it
knows happens-before and modification information for the present, but some
decisions may need to be made in the future as reads-from promises are
fulfilled or modification ordering is observed by future reads and writes.

Lazy checking for the latter cases has yet to be implemented.