feat: implement a basic backend (#1)

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

crates/signed_core/src/addr.rs

@@ -0,0 +1,44 @@
+use std::fmt;
+use std::str::FromStr;
+
+use nostr::prelude::*;
+
+/// Address of a NIP-34 repository announcement: `30617:<owner-pubkey>:<repo-id>`.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct RepoAddr {
+    pub owner: PublicKey,
+    pub id: String,
+}
+
+impl RepoAddr {
+    pub fn new(owner: PublicKey, id: impl Into<String>) -> Self {
+        Self {
+            owner,
+            id: id.into(),
+        }
+    }
+
+    /// The NIP-33 coordinate for the announcement event (`a` tag value).
+    pub fn coordinate(&self) -> Coordinate {
+        Coordinate::new(Kind::GitRepoAnnouncement, self.owner).identifier(self.id.clone())
+    }
+}
+
+impl fmt::Display for RepoAddr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.coordinate())
+    }
+}
+
+impl FromStr for RepoAddr {
+    type Err = nostr::error::Error;
+
+    /// Parse from `<kind>:<pubkey>:<d-tag>`, `naddr1...` bech32 or `nostr:naddr1...` URI.
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let coordinate = Coordinate::parse(s)?;
+        Ok(Self {
+            owner: coordinate.public_key,
+            id: coordinate.identifier,
+        })
+    }
+}