A more number-y Signum

[oscbyspro]

The current Signum model is basically a terse comparison result with some integer-like utilities. It is apparent—since BinaryInteger got operations like as isNegative, isZero, and isPositive—that Signum would be much more interger-like if its cases were negative, zero, and positive. In that case, operations like isNegative, isZero, and isPositive would make sense on Signum too. It might result in less context switching, or something to that effect.

[oscbyspro]

I also suspect that Signum.init(_: Sign?) is preferred over Signum.one(_: Sign).

[oscbyspro]

I'll add a mutating version of negated() since binary integers have negate() now (#80).