Add new fixed-point types

[Frostbyte0x70]

There's multiple places in the repo that refer to various fixed-point value formats, but they are usually documented as integers or shorts. I thought it could be a good idea to document them explicitly, so I defined new types for them. struct fx64 has also been renamed to struct fx64_16 for consistency.

I also plan to make use of these new types in SkyTemple, so I need them to be distinguishable from regular ints and shorts.

[Frostbyte0x70]

I have no idea why that particular run fails, I defined my types next to other common types, so they should all work, right?

[Frostbyte0x70]

I searched for "fixed-point" and updated all symbols that mentioned the fixed-point format, so as long as it was documented in text, it should be updated.