variant backend

Yes, this is crazy, but it means one can traverse the database quickly (basically random access) without having everything in memory.

Funnily enough, the test suite already runs through (the DB building is not hooked up yet). Anyway, mostly posting in case I never touch this again and someone might want to pick it up.

Running gsd-datetime against this drops CPU consumption for startup by about 120ms (down to 40ms). But more importantly, partial access like only needing gweather_location_find_by_country_code need a lot less memory (and basically zero if we add more (transfer full) functions so that the objects can be free'ed again.

TODO:

• Figure out and implement new API
• Install DB into libdir as it is endian dependent
• Fix indentation? I would prefer throwing an automated indenter against all commits, which implies reindenting the world first.