Consider changing the change policy to hide (some) pool-crossing amounts

[str4d]

If a wallet has both Sapling and Orchard notes to use in a transaction, and is able to use the individual pools' notes to pay the necessary Sapling and Orchard outputs, then splitting the change into Sapling and Orchard will avoid any pool crossing (so the only visible amount will be the fee). This adds complexity to note selection and proposal construction, and does not prevent pool crossing in all cases, but we may decide it is worthwhile.