Duration code should not use a trivalued Sign internally

[Manishearth]

While working on #636, I discovered that the nudge and rounding code all uses Sign, which is Positive/Negative/Zero. However, in the spec, it is only allowed to be Positive or Negative. This leads to confusion around the zero case.

We should use a different enum, maybe InternalSign.

[nekevss]

I believe this is addressed by #652.

But that needs to be confirmed.

[Manishearth]

We still use Sign quite a bit, and the spec almost never has a three-value sign. I think the only place we should retain it is in the Duration field (which is public API), though I see good arguments for getting rid of that too.