Req. integer text de/encoding

[oscbyspro]

I think it's reasonable to expect all integers to implement their own text de/encoding methods. It's such an easily forgettable constraint, otherwise. Ideally, these customization points would already be required by BinaryInteger, but they aren't (#60).

[oscbyspro]

By reasonable I mean: Swift already provides this functionality sans customizability.

[oscbyspro]

Correction: Swift's text de/encoding methods are declared as extensions on FixedWidthInteger. I noticed this when I tried to forward a default implementation from ANKBinaryInteger. I suppose I'll do it from ANKFixedWidthInteger instead.