Channel mapping 255 and Ambisonics mapping 2 and 3

[chris-hld]

Ambisonics is currently not available in opusfile.
This PR is aiming to start implementing the remaining mappings for Opus and its tools.

[chris-hld]

The current state should enable encoding and decoding of Ambix / Higher-order ambisonics with the standard opus tools.
Any reviews or suggestions are very welcome.

[olilarkin]

Nice!

[olilarkin]

@chris-hld I am not a maintainer of opusfile, but I imagine the maintainers might like a cleaner commit history on this PR in order to consider it. I am happy to help test this

[chris-hld]

I took care of a last warning and completed the cleanup and documentation / commenting.
Everything related to mapping 3 should now also be guarded by OPUS_HAVE_OPUS_PROJECTION_H.
I am using and testing the modes of this PR daily and at least for me it's all working as expected.

Regarding the commit history, I completely agree. Feel free to squash on merge! If needed I can also rework the history and make it more compact.