Roughly based on https://github.com/godotengine/godot-proposals/issues/3375 (used format is slightly different).
* Implement bitwidth based animation compression (see animation.h for format).
* Can compress imported animations up to 10 times.
* Compression format opens the door to streaming.
* Works transparently (happens all inside animation.h)
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
* Deprecates GDNative in favor of a simpler, lower level interface.
* New extension system allows registering core engine classes.
* Simple header interface in gdnative_interace.h
