Show very little debug output by default.
Fix rmdir.
Allow each storable data type to provide its own marshalling and
unmarshalling format. There is no point in JSON-serializing a data block
because it really only has a single stream of bytes to be stored.

File creation with dd now runs at ~190MB/s vs 80MB/s before, against a
memory-backed store.

Also, we can set up compact in-memory representations of files and
directory metadata entries, which are deterministic in size as opposed
to JSON text.