Single digit multiplication with addition

[oscbyspro]

The unsigned single digit multiplication algorithm can perform one addition for free, so I should probably expose that. I'm unsure about the semantics of the signed version, however, and whether it depends on the representation used. Given a path via the magnitude, it's possible to expose some kind of arcane incrementMagnitudeBy: parameter but I'm not thrilled by it.

// extension NBKDoubleWidth where High == High.Magnitude

mutating func multiply(by other: Digit, add carry: Digit)
func multiplied(by other: Digit, adding carry: Digit) -> Self

mutating func multiplyReportingOverflow(by other: Digit, add carry: Digit) -> Bool
func multipliedReportingOverflow(by other: Digit, adding carry: Digit) -> PVO<Self>

mutating func multiplyFullWidth(by other: Digit, add carry: Digit) -> Digit
func multipliedFullWidth(by other: Digit, adding carry: Digit) -> HL<Digit, Magnitude> 

[oscbyspro]

The combined operation is useful for things like decoding strings.

// text to integer

1. multiply by base
2. add the next digit
3. done or go to 1