merge data/stat records and coalesce writes

This coalesces writes. It's based on merge-stats (#647), so it'll look larger until that branch merges, but you can compare it to that branch to see its specific changes:

https://github.com/mykmelez/j2me.js/compare/merge-stat...coalesce-writes

As expected, it makes the write/flush test (in TestFileSystemPerf) cheap:

> Time to write/flush 1,000 times: 97ms

And fstests.html is also much faster:

> DONE: 127 PASS, 0 FAIL, 125 TIME