Contracts: Refactor API to use WeightMeter (#2943)

Update the Contracts API to use `WeightMeter`, as it simplifies the code
and makes it easier to reason about, rather than taking a mutable weight
or returning a tuple with the weight consumed

---------

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

substrate/frame/contracts/src/migration/v15.rs

@@ -36,6 +36,7 @@ use frame_support::{
 		fungible::{Mutate, MutateHold},
 		tokens::{fungible::Inspect, Fortitude, Preservation},
 	},
+	weights::WeightMeter,
 	BoundedBTreeMap, DefaultNoBound,
 };
 use frame_system::Pallet as System;
@@ -125,7 +126,7 @@ impl<T: Config> MigrationStep for Migration<T> {
 		T::WeightInfo::v15_migration_step()
 	}
 
-	fn step(&mut self) -> (IsFinished, Weight) {
+	fn step(&mut self, meter: &mut WeightMeter) -> IsFinished {
 		let mut iter = if let Some(last_account) = self.last_account.take() {
 			v14::ContractInfoOf::<T>::iter_from(v14::ContractInfoOf::<T>::hashed_key_for(
 				last_account,
@@ -234,10 +235,12 @@ impl<T: Config> MigrationStep for Migration<T> {
 			// Store last key for next migration step
 			self.last_account = Some(account);
 
-			(IsFinished::No, T::WeightInfo::v15_migration_step())
+			meter.consume(T::WeightInfo::v15_migration_step());
+			IsFinished::No
 		} else {
 			log::info!(target: LOG_TARGET, "Done Migrating Storage Deposits.");
-			(IsFinished::Yes, T::WeightInfo::v15_migration_step())
+			meter.consume(T::WeightInfo::v15_migration_step());
+			IsFinished::Yes
 		}
 	}