core: ported regex.gsub to faster native version (#1233)

* added regex.gmatch iterator and other fixes
* fixed issues reported by Guldoman
* push strings with fixed len just in case for binary safety
* added limit to regex.gsub and use pushinteger
* added description to regex.gsub limits param
* replaced substitutions regex description for correctness
* ignore negative limits on regex.gsub

data/core/commands/findreplace.lua

@@ -216,7 +216,7 @@ command.add("core.docview!", {
         return text:gsub(old:gsub("%W", "%%%1"), new:gsub("%%", "%%%%"), nil)
       end
       local result, matches = regex.gsub(regex.compile(old, "m"), text, new)
-      return result, #matches
+      return result, matches
     end)
   end,