Use a lookup table for subject_find_special_char

The special character check runs on most characters in the document, so it's a pretty hot path. By switching to a lookup table we don't have to to scan through each element of `CHARS` on every non-special character.

Performance comparison with 8 copies of the ProGit book concatenated together:

## memchr
    real	0m0.083s
    user	0m0.074s
    sys	0m0.008s

## lookup table
    real	0m0.063s
    user	0m0.040s
    sys	0m0.024s