(context_regex_string, word_regex_string): Remove.

(context_regex_string, word_regex_string): Remove.
(context_regex, word_regex): New vars, replacing the above.
All uses changed.
(struct regex_data): New type.
(compile_regex): Renamed from alloc_and_compile_regex, since
we no longer allocate storage.  Arg is now a struct regex_data *,
not a const char *.  All uses changed.  Don't allocate the fastmap;
instead, take it from the caller.  Don't convert size_t to int,
to avoid arithmetic overflow problems.  Don't bother freeing
storage afterwards; it's not worth the aggravation.