minor style changes in nfa_build_match_byte_range
nfa_build_match_string: if length is -1, use strlen to find length
extract function nfai_byte_match_op to use in build_match_{string,byte}

This means we can be more restrictive about when OP_MATCH_BYTE_CI is used,
so it's only used for alpha characters, and the argument is always stored
in lowercase.
nfai_make_alt: check all parameters
add nfai_is_frag_charclass
merge character classes automatically
update todo list