MemLog.v: use fold_left instead of fold_right in [replay]
MemLog.v: use fold_left instead of fold_right in [replay]

Since our loop iterates over the log in a left-to-right manner,
using [fold_left] makes proof easier w.r.t. [skipn].

Also simpilfies many lemmas about replay.

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