Initial work on flowers.

Need to figure out a way to get faster feedback on changes. Probably some form
of reloading the executable in-place so that data changes have immediate
effect.
Implement serialisation/deserialisation of world.

It definitely isn't suitable for prod use (and I don't think there's any
end-user use for this functionality anyway), but it's fast --- 1s startup if
the dump file is in the OS disk cache.