Duration code should not use a trivalued Sign internally

[Manishearth]

While working on https://github.com/boa-dev/temporal/pull/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.