Allow fee calculation to happen off-chain (#6076)

* Emit a PaymentParameters event once per block

This contains per-block paramaters need to calculate
fees off-chain.

* Add WeightToFee trait

* Add documentation to polynomial types

* Ignore pseudo code snippet for doc tests

* Use `Mul` implementation of Perbill

* Add tests for WeightToFeePolynomial

* Revert "Emit a PaymentParameters event once per block"

This reverts commit 6c4763baff3d8179676a3c1660fe7063fd56a8ca.

Co-authored-by: Gavin Wood <gavin@parity.io>

substrate/bin/node-template/runtime/src/lib.rs

@@ -15,8 +15,7 @@ use sp_runtime::{
 	transaction_validity::{TransactionValidity, TransactionSource},
 };
 use sp_runtime::traits::{
-	BlakeTwo256, Block as BlockT, IdentityLookup, Verify, ConvertInto, IdentifyAccount, NumberFor,
-	Saturating,
+	BlakeTwo256, Block as BlockT, IdentityLookup, Verify, IdentifyAccount, NumberFor, Saturating,
 };
 use sp_api::impl_runtime_apis;
 use sp_consensus_aura::sr25519::AuthorityId as AuraId;
@@ -36,7 +35,7 @@ pub use frame_support::{
 	construct_runtime, parameter_types, StorageValue,
 	traits::{KeyOwnerProofSystem, Randomness},
 	weights::{
-		Weight,
+		Weight, IdentityFee,
 		constants::{BlockExecutionWeight, ExtrinsicBaseWeight, RocksDbWeight, WEIGHT_PER_SECOND},
 	},
 };
@@ -244,7 +243,7 @@ impl transaction_payment::Trait for Runtime {
 	type Currency = balances::Module<Runtime>;
 	type OnTransactionPayment = ();
 	type TransactionByteFee = TransactionByteFee;
-	type WeightToFee = ConvertInto;
+	type WeightToFee = IdentityFee<Balance>;
 	type FeeMultiplierUpdate = ();
 }