Protocol for flexible-width integers

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.