Sneaky signed ± anti-overflow

[oscbyspro]

From oscbyspro/Numberick#3:

The current addition and subtraction methods have this structure:

@inlinable public mutating func addReportingOverflow(_ other: Self) -> Bool {
    let a: Bool = self.low .addReportingOverflow(other.low )
    let b: Bool = self.high.addReportingOverflow(other.high)
    let c: Bool = a && self.high.addReportingOverflow(1 as Digit)
    return b || c // this should be « b != c » (!)
}

The problem is that b and c will negate each other when Self is signed. See regression tests below:

NBKAssertAddition   (T(descending: HL(.max, .max)), T(-1), T(descending: HL(.max, .max - 1)), false) // carry 1st
NBKAssertAddition   (T(descending: HL(.min, .max)), T(-1), T(descending: HL(.min, .max - 1)), false) // carry 2nd

NBKAssertSubtraction(T(descending: HL(.min, .min)), T(-1), T(descending: HL(.min, .min + 1)), false) // carry 1st
NBKAssertSubtraction(T(descending: HL(.max, .min)), T(-1), T(descending: HL(.max, .min + 1)), false) // carry 2nd

These tests validate the case where c appears after b as well as the case where c appears before b.