MD5 hash checking is no longer performed by Nixery (it does not seem
to be necessary), hence the layer cache now only keeps the SHA256 hash
and size in the form of the manifest entry.
This makes it possible to restructure the builder code to perform
cache-fetching and cache-populating for layers in the same place.
The new manifest package creates image manifests and their
configuration. This previously happened in Nix, but is now part of the
server's workload.
This relates to #50.