Compress RDB files

I would like to propose that libretro consider compressing the RDB files. This would require support in libretro to read this format but that should be relatively straightforward. This would be useful for the following reasons:

• It reduces the database size from 117.0 MiB to 34.6 MiB (3.4x reduction) using gzip
• Highly compressible since they contain a lot of duplicate strings and numbers.
• It shrinks the size of the release artifacts for every downstream system (retroarch, batocera, retrobat, rocknix, etc.)
• They're already not editable plaintext so might as well store them as efficiently as possible.
• They're copied to handhelds where storage is a premium that can be used for games or the OS.
• It would reduce the size of the git checkout of this repo.
• Might improve performance slightly since it reduces the amount of data to read and storage not compute is usually the bottleneck

Start by reviewing the RDB files and how libretro and downstream systems currently read them. Define the compressed format, compatibility requirements, and release-artifact changes; the work is done when compressed RDB files are supported without breaking existing consumers and the size reduction is verified.