Add: BinaryInteger/factorial()

[oscbyspro]

Combinations and permutations use factorials; it could be fun. I know little about the state-of-the-art factorial algorithms, but a quick search gives some recursive divide-and-conquer stuff and a bunch of prime swings. I'd rather not prime sieve until I have a scalable sieve approach I'm content with, so maybe I'll try the recursive divide-and-conquer stuff.

[oscbyspro]

The factorial of an infinite binary integer is (value: 0, error: true) because infinite even factors over-shift the result.

[oscbyspro]

I added an over-shift fast path so that silly systems integer factorials don't take too long to compute.