So far I've got the main functionality of register-path working: it
successfully puts the passed information in a database (at least in my
manual tests it did), and it even puts it in the right database based on
prefix and state-directory. But there are quite a few side-effects
missing, most of them noted above register-path in store.scm. There's
deduplication, resetting timestamps and permissions and stuff (the C++
code called it "canonicalizing"), actually returning true in the case of
success like the documentation says it should, processing environment
variables, and figuring out if I should be automatically creating the
database if it doesn't exist yet.

Currently the register-path unit test fails due to being unable to open
the database for writing - the test doesn't specify a prefix or
state-directory, so it's trying to open /var/guix/db/db.sqlite, which
requires root access to open for writing. Perhaps the test is using an
environment variable or similar to specify a different database, which I
haven't implemented yet.

Comments welcome.