Fix gsub, add gsub/3 (fix #782)

tests/all.test

@@ -892,6 +892,10 @@ sub("^(?<head>.)"; "Head=\(.head) Tail=")
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 ["a,b:c:d:e,f",":a,b:c:d:e,f:"]
 
+gsub("(?<d>\\d)"; ":\(.d);")
+"a1b2"
+"a:1;b:2;"
+
 [.[] | scan(", ")]
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 [", ",", ",", ",", ",", ",", ",", ",", "]