Improve fees module (#1821)

* remove amount associated
* make a new trait to bound some arithmetics to balances or assets:
  It also remove arithmetic bounds of srml-support::traits::Currency.

  To update your code then use srml_support::traits::ArithmeticType like:
  `type Currency: ArithmeticType + Currency<Self::AccountId, Balance=BalanceOf<Self>>; ` 
  with `type BalanceOf<T> = <<T as Trait>::Currency as ArithmeticType>::Type; `

* improve decl_storage when it explicit serde bound: basically don't try to be smarter than rust and just use where clause.

substrate/node-template/runtime/src/lib.rs

@@ -165,7 +165,6 @@ impl balances::Trait for Runtime {
 }
 
 impl fees::Trait for Runtime {
-	type Amount = u128;
 	type TransferAsset = Balances;
 	type Event = Event;
 }