This is one way we could organize the flat file storage for blocks - the
alternative would be to not do `type` in the file itself, but have a
single type per file which arguably is simpler but may become annoying.
Another potential restriction would be to require that blocks are
ordered - with this format, it's a little bit more involved to recreate
an index file, and it's easy to accidentally build in assumptions about
the block order in the main data file.
Jacek Sieka