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/srml/support/src/traits.rs

@@ -79,10 +79,14 @@ pub enum UpdateBalanceOutcome {
 	AccountKilled,
 }
 
+pub trait ArithmeticType {
+	type Type: SimpleArithmetic + As<usize> + As<u64> + Codec + Copy + MaybeSerializeDebug + Default;
+}
+
 /// Abstraction over a fungible assets system.
 pub trait Currency<AccountId> {
 	/// The balance of an account.
-	type Balance: SimpleArithmetic + As<usize> + As<u64> + Codec + Copy + MaybeSerializeDebug + Default;
+	type Balance;
 
 	// PUBLIC IMMUTABLES