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/example/src/lib.rs

@@ -281,6 +281,9 @@ mod tests {
 		type OnFreeBalanceZero = ();
 		type OnNewAccount = ();
 		type Event = ();
+		type TransactionPayment = ();
+		type TransferPayment = ();
+		type DustRemoval = ();
 	}
 	impl Trait for Test {
 		type Event = ();