* 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.