tools.lazy-store: unpack NARs with harmonia-file-nar

Our NAR reader goes: NARs are parsed by harmonia-file-nar's parse_nar
and written by its restore (async, bounded queue to one writer thread;
it also checks entry names, sorting and padding like Nix). Files come
out with the umask's permissions, so a pass afterwards sets the store's
0444/0555 like our reader did.

The NAR hash is computed with harmonia-utils-hash (aws-lc sha256) in a
tokio-util InspectReader instead of our HashingReader and sha2, and
narinfo NarHashes are parsed with harmonia (any encoding with the
sha256: prefix) instead of comparing base32/base16 strings. Data after
the NAR counts into the size, so the NarSize check still rejects it.

The .ls listings are harmonia-file-core FileTrees now instead of our
Tree enum converted from them. The test unpacks a NAR made by
harmonia's NarByteStream instead of a hand-written one.