If anyone else is interested in solving this in the short term, this is the extension that I used in a project I'm currently working on:
extension Decimal {
mutating func round(_ scale: Int, _ roundingMode: NSDecimalNumber.RoundingMode) {
var localCopy = self
NSDecimalRound(&self, &localCopy, scale, roundingMode)
}
func rounded(_ scale: Int, _ roundingMode: NSDecimalNumber.RoundingMode) -> Decimal {
var result = Decimal()
var localCopy = self
NSDecimalRound(&result, &localCopy, scale, roundingMode)
return result
}
}