forestdb: Support snapshots and rollbacks, change how with-forestdb works. Unit test things.
forestdb.ffi: Make fdb_iterator_opt_t its own type and remove comment.
forestdb.lib: Implement iterators and some related stuff.
Document possible bugs for now.
forestdb.lib-tests: Test iterators.
forestdb: Add iteration by seq-num. Add tests for iteration by seq-num.
Document potential bugs.