feat: Start dummy in-memory database

src/db/mod.rs

@@ -0,0 +1,11 @@
+mod common;
+pub use common::Database;
+pub mod error;
+mod group;
+pub use group::Group;
+mod interface;
+pub use interface::DatabaseInterface;
+mod memory;
+pub use memory::MemoryDatabase;
+mod user;
+pub use user::{User, UserRef};