db, add core user functionality and tests; db mod

Adds all the basic user functions, and tests for them.

This is also intializes the DB Module, which will have more things in it

koucha/src/db.rs

@@ -0,0 +1,11 @@
+mod user;
+pub use user::User;
+
+macro_rules! define_key {
+  ($name:ident) => {
+    #[derive(PartialEq, Debug, Copy, Clone)]
+    pub struct $name(i64);
+  };
+}
+
+define_key!(UserKey);