Get rid of the tagged OID

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.

src/geef_records.hrl

@@ -1,5 +1,4 @@
 -record(geef_reference, {repo :: pid(), name :: binary(), type :: geef_ref:type(), target :: geef_ref:target()}).
--record(geef_oid, {oid :: binary()}).
 -record(geef_object, {type :: atom(), id :: geef_oid:oid(), handle}).
 -record(geef_index_entry,
 	{ctime :: non_neg_integer(), mtime :: non_neg_integer(),