chore: refactor the backend (#17)

Reviewed-on: #17

crates/signed_core/src/lib.rs

@@ -11,6 +11,9 @@ pub use addr::{RepoAddr, identifier_from_name, repo_addr};
 pub use annotations::{COVER_NOTE_KIND, cover_note, labels_and_subject, subject_override};
 pub use clone_url::{CloneTarget, parse_clone_url};
 pub use deletions::Deletions;
-pub use model::{Announcement, activity_subject, pull_request_patch, pull_request_patches};
+pub use model::{
+    Announcement, activity_subject, branch_name_of, clone_urls_of, current_commit_of,
+    fork_candidates, latest_update, merge_base_of, pull_request_patch, pull_request_patches,
+};
 pub use state::{build_state, parse_state};
 pub use status::{RepoStatus, references_root, resolve_status};