`txouts` and `txouts_in_tx` are exposed from `SpkTxOutIndex`, but modified to remove nested unions. Add `keychain_outpoints_in_range` that iterates over outpoints of a given keychain derivation range.
`txouts` and `txouts_in_tx` are exposed from `SpkTxOutIndex`, but modified to remove nested unions. Add `keychain_outpoints_in_range` that iterates over outpoints of a given keychain derivation range.