Clean up lualibs

Clean up the modules in engines/LuaCommon/lualib:
* Fix luabit/bit.lua to return its table instead of trying to set the
  global directly.
* Fix luabit/hex.lua to return its table instead of trying to set the
  global directly.
* luabit/noki.lua is useless for our purposes
* luabit/utf8.lua is redundant to mw.ustring
* stringtools uses coroutines, which we don't support

Also fix a parser test that has apparently been broken for a long time.

Change-Id: I1284cddb6e9b94327964cb1077d8dbdf7def6d06