Keep references purely in erlang

Refernces contain three pieces of data which we already keep in the
erlang record. Keep the repository it belongs to in the record as well
and remove resource in the NIF. This makes the code easier to reason
about, as we don't need to keep any reference counts or GC the C
struct.

This does mean we cannot re-use references across geef_repo restarts,
but that's probably a good thing.
Make sure to test geef_ref:resolve()
Merge branch 'pure-ref'