Use typedefs for strings, symbols, keywords, tuples, structs, and abstracts.

[wooosh]

This PR replaces generic const uint8_t * and Janet * types with the more specific JanetString/JanetSymbol/JanetKeyword/JanetTuple/JanetStruct/JanetAbstract.

This makes it easier for those who are new to the C API (such as myself) to figure out what these functions actually return/take as arguments. I specifically had issues understanding how the janet_getstring function worked initially because it appeared to return a pointer to the start of the string without a length.

AFAICT the only reason these functions do not use the more specific types is because their function signatures predate the typedefs existing.

I'll send a PR to update the corresponding docs at https://janet-lang.org if this gets merged.

[primo-ppcg]

I can't say if there's a reason why this wasn't done, although it seems reasonable to me.

I think you may have overlooked a few void * to JanetAbstract substitutions:

701913f#diff-6b11ee3d77a81a7688781e2c4013c332149289852e0369324b6797a5719ae364L500
701913f#diff-5f98ee242ca534971f89d34bc88e916059cfe3b7a9e240a843213e987f88b061L657
701913f#diff-5f98ee242ca534971f89d34bc88e916059cfe3b7a9e240a843213e987f88b061L788
701913f#diff-5f98ee242ca534971f89d34bc88e916059cfe3b7a9e240a843213e987f88b061L837
701913f#diff-5f98ee242ca534971f89d34bc88e916059cfe3b7a9e240a843213e987f88b061L860

[wooosh]

They can't be made into JanetAbstract because they are const pointers. const JanetAbstract becomes the type void * const instead of the desired const void * because C's const specifier is token based. It would be nice to be able to include these, but I'm not sure how I would implement it.