Add `FixedPointOperand` blanket implementation (#14634)

* bound `Balance` to `FixedPointOperand`

* ".git/.scripts/commands/fmt/fmt.sh"

* clean up code

* Apply suggestions from code review

Co-authored-by: Gavin Wood <[email protected]>

* wip

* add blanket `FixedPointOperand` impl

* update nis CurrencyBalance

* remove CheckedNeg bound

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Co-authored-by: command-bot <>
Co-authored-by: Gavin Wood <[email protected]>