MINOR: hlua_fcn: wrap pat_ref struct for patref class

MINOR: hlua_fcn: wrap pat_ref struct for patref class

In order to extend the patref class features, let's wrap the pat_ref struct
into hlua_patref struct. This way we may add additional data alongside the
pat_ref pointer to store additional context required for pat_ref data
manipulation from lua.

Since the wrapper (hlua_patref) is an allocated object, we declare the _gc
metamethod for patref class in order to properly cleanup resources when
they are out of scope.