Add: BinaryInteger/isqrt()

[oscbyspro]

I'll add some binary integer square root methods.

[oscbyspro]

Hm. I'm not sure the extensions on Nonzero<T> are worth the headache. I don't really want to juggle guarantee types (#62). It might be better to just pick the most lenient set that promises nonoptional results and stick with it. In this case, that would be Natural<T>. It's an extra is-zero check, but it's also a more straightforward type conversion story. Natural<T> might be the best fit even if I had some kind of Nonzero<Natural<T>> type (#78), in part because the Nonzero<T> hoist could become redundant if I change the algorithm.

[oscbyspro]

The internal UnsignedInteger.isqrt(natural: Nonzero<Self>) helper method approach is a bit faster, for reasons unclear to me, even when the is-zero check is performed. But, getting the function count down to 4 (from 6) is nice.