Provide rust code to pack/unpack data compatible with BitUnPack

[Lokathor]

It would be very nice if we had some pure rust code that could pack data into the BitUnPack format. This could be called by a build script, CLI tool, etc.

Also we might as well have a pure rust unpacking function too.

An initial impl can just live in this crate, but perhaps if we get code for all three GBA compression formats working well then that could be moved into its own crate.