Introduce safe types for handling imbalances (#2048)

* Be a little safer with total issuance.

* PairT instead of _Pair

* Remove rev causing upset

* Remove fees stuff.

* Fix build (including tests)

* Update runtime, bump version

* Fix

* Handle gas refunds properly.

* Rename identifier

ala #2025

* Address grumbles

* New not-quite-linear-typing API

* Slimmer API

* More linear-type test fixes

* Fix tests

* Tidy

* Fix some grumbles

* Keep unchecked functions private

* Remove another less-than-safe currency function and ensure that
contracts module can never create cash.

* Address a few grumbles and fix tests

substrate/srml/staking/src/mock.rs

@@ -58,6 +58,9 @@ impl balances::Trait for Test {
 	type OnFreeBalanceZero = Staking;
 	type OnNewAccount = ();
 	type Event = ();
+	type TransactionPayment = ();
+	type TransferPayment = ();
+	type DustRemoval = ();
 }
 impl session::Trait for Test {
 	type ConvertAccountIdToSessionKey = ConvertUintAuthorityId;
@@ -72,6 +75,8 @@ impl Trait for Test {
 	type Currency = balances::Module<Self>;
 	type OnRewardMinted = ();
 	type Event = ();
+	type Slash = ();
+	type Reward = ();
 }
 
 pub struct ExtBuilder {
@@ -193,6 +198,8 @@ impl ExtBuilder {
 					(40, balance_factor), (41, balance_factor * 40)
 				]
 			},
+			transaction_base_fee: 0,
+			transaction_byte_fee: 0,
 			existential_deposit: self.existential_deposit,
 			transfer_fee: 0,
 			creation_fee: 0,