btrfs-disk: on-disk format parsing for btrfs filesystems

This crate reads and parses btrfs on-disk structures directly from block devices or image files, without going through the kernel. It covers superblock parsing, and will eventually include tree node and leaf parsing.

Unlike btrfs-uapi (which wraps Linux-only ioctls), this crate is platform-independent: any system that can read raw bytes from a block device or image file can use it.