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/core/primitives/src/ed25519.rs

@@ -523,7 +523,7 @@ impl Pair {
 mod test {
 	use super::*;
 	use hex_literal::{hex, hex_impl};
-	use crate::{Pair as _Pair, crypto::DEV_PHRASE};
+	use crate::{Pair as PairT, crypto::DEV_PHRASE};
 
 	#[test]
 	fn default_phrase_should_be_used() {

substrate/core/primitives/src/sr25519.rs

@@ -501,7 +501,7 @@ impl Pair {
 #[cfg(test)]
 mod test {
 	use super::*;
-	use crate::{Pair as _Pair, crypto::{Ss58Codec, DEV_PHRASE, DEV_ADDRESS}};
+	use crate::{Pair as PairT, crypto::{Ss58Codec, DEV_PHRASE, DEV_ADDRESS}};
 	use hex_literal::{hex, hex_impl};
 
 	#[test]