Revamp sub/3 to resolve most issues with gsub (and sub with "g") (#2641)

The primary purpose of this commit is to rectify most problems with
`gsub` (and also `sub` with the `g` option), in particular fix #1425 ('\b'),
fix #2354 (lookahead), and fix #2532 (regex == `"^(?!cd ).*$|^cd "`).

This commit also partly resolves #2148 and resolves #1206 in that
`gsub` no longer loops infinitely; however, because the new `gsub`
depends critically on `match/2`, the behavior when regex == `""` is
sometimes non-standard.

The documentation has been updated to reflect the fact that `sub`
and `gsub` are intended to be regular in the second argument.

Also, `_nwise/1` has been tweaked to take advantage of TCO.

src/builtin.c

@@ -930,7 +930,8 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
         match = jv_object_set(match, jv_string("string"), jv_string(""));
         match = jv_object_set(match, jv_string("captures"), jv_array());
         result = jv_array_append(result, match);
-        start += 1;
+        // ensure '"qux" | match("(?=u)"; "g")' matches just once
+        start = (const UChar*)(input_string+region->end[0]+1);
         continue;
       }