This means we no longer accept any iolist in some of the functions,
but we might bring it back later.
There is also a new function, geef_ref:create_symbolic() to create
symbolic references, since we can no longer differentiate between a
name and an OID.
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.
Use erlang's allocator and simplify the code by writing into the
buffer we're going to give back anyway.
Also introduce a function to NUL-terminate iolist buffers, which can
avoid memory copying.
