Privately patching strings lib in Lua 5.2

[Andrew Starks <andrew.starks@...>]

```lua
_ENV = {
  print = print, string = string, table = table, pairs = pairs, type = type
}

do
  local _string = string
  local concat = table.concat
  string = {}
  for m_name, m_func in pairs(_string) do
    string[m_name] = m_func
  end

  function string:map(...)
    local args = type(...) == "table" and (...) or {...}

    return self .. concat(args, self)

  end


end
--testing
--This is result that I want.
print(string.map("test","this", "that"))
-->testthistestthat

--This is the syntax that I want to use, but it doesn't work...
print(("test"):map("this", "that"))
--lua: ...src\testing\test_string_library_question.lua:23: attempt to
call method 'map' (a nil value)
```
If I skip the parts where I hide my change from the global
environment, then both function calls behave identically.

Is there a way to patch the `string` library so that the changes apply
to strings using the `self`/colon method, without patching `string`
globally? Debug library?

--Andrew