Also, use ‘idx_t’ for ptrdiff_t values that must be nonnegative,
but do this only for internal use for now.
* NEWS: Mention the API change.
* lib/dfa.c (idx_t, IDX_MAX): New type and max value, for internal
use for now. Use them instead of ptrdiff_t and PTRDIFF_MAX for
values known to be nonnegative.
(dfaparse, dfaexec_mb, dfaexec_sb, dfaexec_noop, dfaexec):
Prefer idx_t or ptrdiff_t to size_t for API.
* lib/dfa.h (dfaparse, dfacomp, dfaexec):
Prefer ptrdiff_t to size_t for API.