* Open all database tables (Db objects) at initialisation time, not
every time they are used. This is necessary because tables have to
outlive all transactions that refer to them.
This is not entirely trivial since this introduces the possibility
of mutual recursion.
* Made normal forms self-contained.
* Use unique ids, not content hashes, for content referencing.
archives (using the package in corepkgs/nar).
* queryPathByHash -> expandHash, and it takes an argument specifying
the target path (which may be empty).
* Install the core Fix packages in $prefix/share/fix. TODO: bootstrap
Nix and install Nix as a Fix package.
