Mostly cosmetic changes to GEOS C API

- Replace "reader" with "writer" in prototypes
- describe "_r" functions (this could possibly be rewritten better than my attempt)
- Function arguments with only one geometry changed from `g1` to `g`
- Rename `nf` -> `ef` for `GEOSContext_setErrorHandler_rsetErrorHandler_r`
- Move `GEOSBuffer*` declaration to top of buffer related function section
- Change `char* mat` to `char *mat`
- Fix typos in comments
- Clip trailing white space, and other white space consistencies
- Other minor rearrangements for consistency