Add reference type and make_ref call

[filmor]

This is a first step towards #95.

enif_make_ref is the only direct rustler_sys reference I could find on github outside of rustler itself (in extorch). Since #653 is a potentially breaking change for direct users of rustler_sys (at least on Windows, where the initialisation won't get called anymore), this code needs to be adjusted on upgrade anyhow, so it makes sense to add the used functionality directly.