Protocol for flexible-width integers

[oscbyspro]

I'll need a flexible-width integer protocol, in order for NBKSigned<UIntXL> to function as a "BigInt" model (#33) (#81). NBKSigned<Magnitude> is not a binary integer, so it won't conform to it, but it needs to know when the Magnitude does so that it can implement things like add-at-index and whatnot.