MemoServ: add CANCEL, CHECK, and INFO

CANCEL recalls the sender's most recent unread memo to a target (one the
recipient already read can't be recalled); CHECK reports whether the last
memo you sent someone has been read; INFO summarises your mailbox. Adds
memo_cancel/memo_check store methods and shares MAX_MEMOS from the module
root instead of duplicating it in send.rs.

modules/memoserv/src/info.rs

@@ -0,0 +1,12 @@
+use echo_api::{Sender, ServiceCtx, Store};
+
+// INFO: a summary of your mailbox (total, unread, capacity).
+pub fn handle(me: &str, from: &Sender, account: &str, ctx: &mut ServiceCtx, db: &mut dyn Store) {
+    let total = db.memo_list(account).len();
+    let unread = db.unread_memos(account);
+    ctx.notice(
+        me,
+        from.uid,
+        format!("You have \x02{total}\x02 memo(s), \x02{unread}\x02 unread, of a maximum \x02{}\x02.", super::MAX_MEMOS),
+    );
+}