File IO improvements in .NET 6

Shared for the technical content and not because I think there are that many .NET devs on HN.

Aside from the gems, some of the things in there are real facepalm moments that would never have happened in the original IO code if the team that came up with it even communicated with the teams responsible for the Windows IO subsystem. I’d (wrongly) assumed that file IO in .NET was not much more expensive than doing it yourself in unmanaged code like most of us were used to doing on Windows, but that would have been a very dangerous assumption.