24-bit integer types in stdint.h

[mysterymath]

It looks (cursorily) like we're within our rights to establish 24-bit integer types in stdint.h. The _BitInt(24) extension for this isn't very well known, and it's come up a few times that this is a desirable type, so we should look into making it work the same way as the other stdint types.

[mysterymath]

It looks like this comes with the additional requirement to specify format strings for these types in inttypes.h. That will be difficult without getting really creative with the printf format string; we can probably do it, but it will be difficult. Accordingly, bumping this down to p3; _Bitint suffices for now, as it makes it explicit that it's an extension, not a natively-supported integer type as defined by the C standard.