Generic Normalize impl for arithmetic and npos-elections (#6374)

* add normalize

* better api for normalize

* Some grumbles

* Update primitives/arithmetic/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* More great review grumbles

* Way better doc for everything.

* Some improvement

* Update primitives/arithmetic/src/lib.rs

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

substrate/frame/staking/src/offchain_election.rs

@@ -203,7 +203,8 @@ pub fn prepare_submission<T: Trait>(
 	}
 
 	// Convert back to ratio assignment. This takes less space.
-	let low_accuracy_assignment = sp_npos_elections::assignment_staked_to_ratio(staked);
+	let low_accuracy_assignment = sp_npos_elections::assignment_staked_to_ratio_normalized(staked)
+		.map_err(|e| OffchainElectionError::from(e))?;
 
 	// convert back to staked to compute the score in the receiver's accuracy. This can be done
 	// nicer, for now we do it as such since this code is not time-critical. This ensure that the

substrate/frame/staking/src/testing_utils.rs

@@ -201,11 +201,8 @@ pub fn get_weak_solution<T: Trait>(
 	};
 
 	// convert back to ratio assignment. This takes less space.
-	let low_accuracy_assignment: Vec<Assignment<T::AccountId, OffchainAccuracy>> =
-		staked_assignments
-			.into_iter()
-			.map(|sa| sa.into_assignment(true))
-			.collect();
+	let low_accuracy_assignment = assignment_staked_to_ratio_normalized(staked_assignments)
+		.expect("Failed to normalize");
 
 	// re-calculate score based on what the chain will decode.
 	let score = {