lib/repo-refs: Add first version of ostree_repo_resolve_collection_ref()

This is a parallel for ostree_repo_resolve_rev_ext() which works on
collection–refs. At the moment, the implementation is simple and uses
ostree_repo_list_collection_refs(). In future, it could be rewritten to
check the checksum directly rather than enumerating all
potentially-relevant checksums.

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

Closes: #1182
Approved by: cgwalters

src/libostree/libostree-experimental.sym

@@ -77,3 +77,8 @@ global:
   ostree_repo_transaction_set_collection_ref;
   ostree_validate_collection_id;
 } LIBOSTREE_2017.7_EXPERIMENTAL;
+
+LIBOSTREE_2017.12_EXPERIMENTAL {
+global:
+  ostree_repo_resolve_collection_ref;
+} LIBOSTREE_2017.8_EXPERIMENTAL;