WIP: Implement writes.
WIP: Basic read implementation.
WIP: Figuring out why File instance dies (has nil entries) if we wait
for a few minutes after the previous operation.

Reproduce by creating a new file, waiting for 2 minutes and then reading
the same file. Should crash BuddyFS.

Culprit: Forget. The file handle is invalidated and corresponding
instance is unusable.
Unbreak tests.
Basic tests for memstore.