Transaction Fee Module (#1648)

* wip

* Split bytes fee charging and charging by amount into different traits.

* Move to edition 2018.

* Implemented charge fee traits for fees module.

* Implemented 'on_finalise' for fee module.

* Updated fees finalize impl.

* Renaming and documentation update.

* Added overflow & underflow check for fee calculation.

* Added mock and unit tests for fee module.

* More unit tests for fees module.

* Fixed srml-executive unit tests.

* Remove transaction base/bytes fee from balances module, fix unit tests.

* fix compile error

* Fixed unit test.

* Minor fixes.

* Bump spec version.

* Bump spec version.

* Updated fees module and runtime wasm.

* Fees module code style improvement; updated runtime wasm.

* Bump spec and impl version.

substrate/srml/staking/src/mock.rs

@@ -104,8 +104,6 @@ pub fn new_test_ext(
 		} else {
 			vec![(10, balance_factor), (20, balance_factor)]
 		},
-		transaction_base_fee: 0,
-		transaction_byte_fee: 0,
 		existential_deposit: ext_deposit,
 		transfer_fee: 0,
 		creation_fee: 0,