feat: Start dummy in-memory database

src/db/interface.rs

@@ -0,0 +1,17 @@
+use crate::db::error::*;
+use crate::db::{Database, User, UserRef};
+
+impl<D: DatabaseInterface> DatabaseInterface for Database<D> {
+    async fn get_user(&self, user: &UserRef) -> Option<User> {
+        self.inner.read().await.get_user(user).await
+    }
+
+    async fn create_user(&mut self, user: User) -> Result<(), UserAlreadyExists> {
+        self.inner.write().await.create_user(user).await
+    }
+}
+
+pub trait DatabaseInterface {
+    async fn get_user(&self, user: &UserRef) -> Option<User>;
+    async fn create_user(&mut self, user: User) -> Result<(), UserAlreadyExists>;
+}