parse-datetime: Use idx_t for nonnegative ptrdiff_t variables.

parse-datetime: Use idx_t for nonnegative ptrdiff_t variables.

* lib/parse-datetime.y: Include idx.h.
(textint): Mark digits as nonnegative.
(parser_control): Mark dates_seen, days_seen, local_zones_seen,
dsts_seen, times_seen, zones_seen as nonnegative.
(lookup_word): Mark wordlen as nonnegative.
(yylex): Mark count as nonnegative.
(parse_datetime2): Mark tzsize as nonnegative.
* modules/parse-datetime (Depends-on): Add idx.