I have no idea what other osu! file format parsers there are, but this one is mine!

The focus here was on performance so I tried to do 0-alloc deserialization where possible. I've tested roundtrips on a lot of beatmaps with different format versions so I'm reasonably sure this should cover most of them (roundtrips aren't byte-by-byte perfect all of the time because there's lots of leeway, but it's reasonably close at least).

Sorry for having very sparse documentation, I'll improve it later... hopefully