feat: implement a basic backend (#1)

Reviewed-on: https://git.reya.su/reya/signed/pulls/1

crates/signed_core/src/lib.rs

@@ -0,0 +1,10 @@
+pub mod addr;
+pub mod clone_url;
+pub mod filters;
+pub mod model;
+pub mod status;
+
+pub use addr::RepoAddr;
+pub use clone_url::{CloneTarget, parse_clone_url};
+pub use model::Announcement;
+pub use status::{RepoStatus, references_root, resolve_status};