macos-btrfs port

This work-in-progress project aims to port the BTRFS filesystem from the Linux kernel into a usable MacOS kernel extension, seamlessly integrated into diskutil. It's released under the GNU GPL v3, and much of it is based on the Apple ntfs extension source code, the Linux Kernel implementation of BTRFS, and WinBtrfs.

Currently, this does not mount, as the vfs_mount functions haven't been implemented yet; as a small side project, development on this progresses in turtle years -- and only when I have free time (which tends to be never). If you'd like to contribute, please feel free to submit a PR.

** Do not use this on a live system, or your kernel will crash.** If you're interested in testing and debugging this kext, a VM is recommended.