Call ffi.typeof() directly

[AltraMayor]

luaL_get_ctypeid(), which GT blocks call multiple times while interacting with the Lua policy (see #210 for a reference), gets a reference to ffi.typeof() running the Lua code return require('ffi').typeof. Most of this overhead goes away if someone finds out how to call the C function of LuaJIT under ffi.typeof().