Future-proof core integer dividingFullWidthReportingOverflow(_:)

[oscbyspro]

From oscbyspro/Numberick#18

I'm not sure I'd consider this a bug, because it works, but it can be improved. Swift's documentation states that dividingFullWidth(_:) may cause a run-time error in the case of overflow. Well, it doesn't, and I'm using the current behavior in my implementation of [...]ReportingOverflow(_:). Here's the relevant line, but not the whole method:

var qro = NBK.bitCast(PVO(rhsMagnitude.dividingFullWidth(lhsMagnitude), lhsMagnitude.high >= rhsMagnitude)) as PVO<QR<Self, Self>>

While this is correct in Swift 5.8, it may or may not be future-proof. I could reimplement the entire dividing thingy, but that's a pain. Alternatively, I can do the truncation myself then pass the value to dividingFullWidth(_:).