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/node-template/runtime/Cargo.toml

@@ -15,7 +15,6 @@ version = { package = "sr-version", path = "../../core/sr-version", default_feat
 support = { package = "srml-support", path = "../../srml/support", default_features = false }
 primitives = { package = "substrate-primitives", path = "../../core/primitives", default_features = false }
 balances = { package = "srml-balances", path = "../../srml/balances", default_features = false }
-fees = { package = "srml-fees", path = "../../srml/fees", default_features = false }
 consensus = { package = "srml-consensus", path = "../../srml/consensus", default_features = false }
 aura = { package = "srml-aura", path = "../../srml/aura", default_features = false }
 executive = { package = "srml-executive", path = "../../srml/executive", default_features = false }
@@ -37,7 +36,6 @@ std = [
 	"runtime-io/std",
 	"support/std",
 	"balances/std",
-	"fees/std",
 	"executive/std",
 	"aura/std",
 	"indices/std",