Add a new FFI-based decoder for GR2 files

[rdw-software]

Roadmap:

• [x] Implement a basic GR2 decoder (based on the OpenGR2 spec, this should be easy)
• [ ] Deal with the Oodle compression, somehow (the most difficult part)
• [ ] Implement the virtual pointer resolution (TBD - see OpenGR2 wiki)
• [ ] Process the actual data tree (TBD: What? How? Expected result?)
• [ ] Export Bink textures (see this description? TBD: Not sure if the format is the same)
• [ ] Export animation data (samples or splines, not sure which is easier to use)

[rdw-software]

Interestingly, a detailed write-up/eference implementation now exists for OODLE-1: https://github.com/LunaticInAHat/liboodle

Clearly, there are many differences in the "inner" parts of the arithmetic decoder; it seems OODLE-0 is simpler than OODLE-1:

• Instead of LZSS, it uses a standard LZ77 "sliding window copy" mechanism (the emitted tokens being length/distance pairs)
• Match length encoding appears to be just as described in the above document, no changes here AFAICT (at first glance)
• Compressor settings (called "Oodle1 header" in this section) are probably the same, except backrefs are split differently
• The compressed bitstream directly maps to the encoded interval - I think? (No shenanigans, except it's dumped in reverse)
• Not sure about the learning/model, but it should be possible to dump the generated frequency tables (for further analysis)

Although dealing with GR2 files is super low priority, it may be feasible to decode them if the tables can be reconstructed OTF.