Generic exponent in BinaryInteger/power(_:)

[oscbyspro]

(#67) made me wonder whether the exponent (#53) type should be UX or Magnitude. But, making it fully generic would be even better. It is important that each specialization uses the same underlying implementation and there are multiple ways of ensuring that. I believe, but I'll have to double-check, that the truncated value repeats itself after some power-of-two exponent less than or equal to pow(2, size). If that's the case, then I could simply load the bit pattern that fits in the greater of UX and Magnitude and perform some minor error handling if it doesn't. Otherwise, maybe I'll use a bit-view thingy.

[oscbyspro]

Seems like odd exponents repeat whereas even exponents produce zeros.

[oscbyspro]

I wonder if I should use UX when the Magnitude is small to maximize the chance that the generic exponent fits, or if it is better to use a smaller type and take the does-not-fit path.

[oscbyspro]

I8(123).power(U8 .max) // Fallible<I8>(value: -77, error: true)
I8(123).power(U16.max) // Fallible<I8>(value: -77, error: true)
I8(123).power(U32.max) // Fallible<I8>(value: -77, error: true)
I8(123).power(U64.max) // Fallible<I8>(value: -77, error: true)
I8(123).power(UXL.max) // Fallible<I8>(value: -77, error: true)

[oscbyspro]

I wish it were possible to annotate a generic type parameter with a preferred concrete type.