Improve OnUnbalance trait for better information on the unbalanced amounts

[gui1117]

Idea from @bkchr paritytech/substrate#8860 (comment)

Allow Unbalance to give a concrete type holding the unbalanced amounts:

Something like:

trait OnUnbalanced<V, R> where R: IntoIterator<Item=V> {
     fn on_unbalanceds<B>(whatever: R) {}
}

For example in transaction-payment we could introduce the struct:

struct TransactionPayment {
    pub fees,
    pub tips,
}

and require an implementation of OnUnbalanced<V, TransactionPayment>

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.

[gui1117]

that would still be an improvment of the API