lib/refs: Add methods for setting/listing collection–refs

These are tuples of (collection ID, ref name) which are a globally-unique
form of local ref. They use OstreeCollectionRef as an identifier, and hence
need to be accessed using new API, as the existing API uses string
identifiers and sometimes accepts refspecs. Remote names are not
supported as part an OstreeCollectionRef.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #924
Approved by: cgwalters

src/libostree/libostree-experimental.sym

@@ -46,6 +46,9 @@ global:
   ostree_collection_ref_hash;
   ostree_collection_ref_new;
   ostree_repo_get_collection_id;
+  ostree_repo_list_collection_refs;
   ostree_repo_set_collection_id;
+  ostree_repo_set_collection_ref_immediate;
+  ostree_repo_transaction_set_collection_ref;
   ostree_validate_collection_id;
 } LIBOSTREE_2017.7_EXPERIMENTAL;