File system: we now have a no_std-friendly btrfs crate

[kennystrawnmusic]

Browsing crates.io for file system crates turned up something very interesting: a btrfs driver which, upon snooping the code, was very easy to port to make it work in #![no_std] environments. So I forked it to accomplish exactly that.

Not thoroughly tested of course, but this would definitely be a great choice for integration into the bootloader so that it recognizes btrfs as a file system from which to load a kernel binary (something very notably on the checklist at the bottom of the README).