The immediate motivation is that Cargo.lock referred to a commit version
in a PR branch of my nix fork that no longer exists. (I didn't know, but
it makes sense, that "git push -f" not only forcibly updates the branch
to refer to a new commit but also gets rid of orphaned commits.) Use a
moonfire branch that I'll keep stable until I'm ready to move on.
I also updated parking_lot and rusqlite to new major versions (nothing
in the interface that I care about has changed) and did a full cargo
update.
This is nicer in a few ways:
* I can use openat so there's no possibility of any kind of a race
involving scanning a different directory than the one used in
other ways (locking, metadata file, adding/removing sample files)
* filename() doesn't need to allocate memory, so it's a bit more
efficient
* dogfooding - I wrote nix::dir.
Add a new schema version 5; now 4 means the directory meta may or may
not be upgraded.
Fixes#65: now it's possible to open the directory even if it lies on a
completely full disk.
