opti(ud): mul instead of adds for creating UD

Single multiplication instead of N additions for computing the monetary mass delta.

@librelois

pallets/universal-dividend/src/lib.rs

@@ -191,15 +191,14 @@ pub mod pallet {
             let total_weight: Weight = 0;
 
             let ud_amount = <CurrentUdStorage<T>>::try_get().expect("corrupted storage");
-            let mut monetary_mass = <MonetaryMassStorage<T>>::try_get().expect("corrupted storage");
+            let monetary_mass = <MonetaryMassStorage<T>>::try_get().expect("corrupted storage");
 
             for account_id in T::MembersIds::get() {
                 T::Currency::deposit_creating(&account_id, ud_amount);
-                monetary_mass += ud_amount;
                 Self::write_ud_history(n, account_id, ud_amount);
             }
 
-            <MonetaryMassStorage<T>>::put(monetary_mass);
+            <MonetaryMassStorage<T>>::put(monetary_mass + ud_amount * members_count);
             Self::deposit_event(Event::NewUdCreated(ud_amount, members_count));
 
             total_weight