radicle: Add `resolves` and `labels` method to `cob::patch::Revision` — heartwood

To be able to query the BTreeSet that shows which comments are resolved by a specific revision and the Iterator of Label we require some methods to obtain them.

         &self.discussion
     }
     /// Review comments resolved by this revision.
     pub fn resolves(&self) -> &BTreeSet<(EntryId, CommentId)> {
         &self.resolves
     }
     /// Iterate over all top-level replies.
     pub fn replies(&self) -> impl Iterator<Item = (&CommentId, &thread::Comment<CodeLocation>)> {
         self.discussion.comments()

         self.comments.comments()
     }
     /// Review labels.
     pub fn labels(&self) -> impl Iterator<Item = &Label> {
         self.labels.iter()
     }
     /// Review general comment.
     pub fn summary(&self) -> Option<&str> {
         self.summary.as_deref()