reorganize into separate crates

- taskchampion -- core implementation of a replica
 - taskchampion-cli -- command-line interface
 - taskchampion-sync-server -- server implementation (not much yet!)

taskchampion/src/server.rs

@@ -0,0 +1,20 @@
+pub type Blob = Vec<u8>;
+
+pub enum VersionAdd {
+    // OK, version added
+    Ok,
+    // Rejected, must be based on the the given version
+    ExpectedVersion(u64),
+}
+
+/// A value implementing this trait can act as a server against which a replica can sync.
+pub trait Server {
+    /// Get a vector of all versions after `since_version`
+    fn get_versions(&self, username: &str, since_version: u64) -> Vec<Blob>;
+
+    /// Add a new version.  If the given version number is incorrect, this responds with the
+    /// appropriate version and expects the caller to try again.
+    fn add_version(&mut self, username: &str, version: u64, blob: Blob) -> VersionAdd;
+
+    fn add_snapshot(&mut self, username: &str, version: u64, blob: Blob);
+}