Use stock LuaJIT

It may be possible to review Gatekeeper's interface between C and LuaJIT to remove the need for `luaL_pushcdata()`, `luaL_checkcdata()`, and `luaL_get_ctypeid()`; these are the only functions required to integrate Gatekeeper and LuaJIT.

It seems that other projects have a similar need for those functions, so LuaJIT might eventually come up with a solution. If so, we'd be better off moving to the new solution.