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.

tests/onig.test

@@ -75,6 +75,45 @@ gsub( "(.*)"; "";  "x")
 ""
 ""
 
+gsub( ""; "a";  "g")
+""
+"a"
+
+gsub( "^"; "";  "g")
+"a"
+"a"
+
+
+# The following is a regression test and should not be construed as a requirement other than that execution should terminate:
+gsub( ""; "a";  "g")
+"a"
+"aa"
+
+gsub( "$"; "a";  "g")
+"a"
+"aa"
+
+gsub( "^"; "a")
+""
+"a"
+
+gsub("(?=u)"; "u")
+"qux"
+"quux"
+
+gsub("^.*a"; "b")
+"aaa"
+"b"
+
+gsub("^.*?a"; "b")
+"aaa"
+"baa"
+
+# The following is for regression testing and should not be construed as a requirement:
+[gsub("a"; "b", "c")]
+"a"
+["b","c"]
+
 [.[] | scan(", ")]
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 [", ",", ",", ",", ",", ",", ",", ",", "]
@@ -92,7 +131,33 @@ gsub("(?<x>.)[^a]*"; "+\(.x)-")
 "Abcabc"
 "+A-+a-"
 
+gsub("(?<x>.)(?<y>[0-9])"; "\(.x|ascii_downcase)\(.y)")
+"A1 B2 CD"
+"a1 b2 CD"
+
+gsub("\\b(?<x>.)"; "\(.x|ascii_downcase)")
+"ABC DEF"
+"aBC dEF"
+
 # utf-8
 sub("(?<x>.)"; "\(.x)!")
 "’"
 "’!"
+
+[sub("a"; "b", "c")]
+"a"
+["b","c"]
+
+[sub("(?<a>.)"; "\(.a|ascii_upcase)", "\(.a|ascii_downcase)", "c")]
+"aB"
+["AB","aB","cB"]
+
+[gsub("(?<a>.)"; "\(.a|ascii_upcase)", "\(.a|ascii_downcase)", "c")]
+"aB"
+["AB","ab","cc"]
+
+# splits and _nwise
+[splits("")]
+"ab"
+["","a","b"]
+