signed_duration: add round method

[BurntSushi]

This adds a new SignedDuration::round method that behaves like Span,
but for time (non-calendar) units only.

For the most part, we just copy Span::round as-is, but without the
largest unit knob (which doesn't make sense for something like
SignedDuration) or the relative datetime knob (which isn't needed
for a duration with all invariant units, as is the case for
SignedDuration). We can even reuse the same implementation for Span
rounding: we convert the duration to a 128-bit signed integer of
nanoseconds, do the rounding and then convert back (and check for
overflow).

Note that we specifically don't both with adding rounding functionality
built into APIs like Zoned::duration_since or
civil::DateTime::duration_until, unlike what we do for Span, since
composition works better for SignedDuration than it does for Span.
Namely, in order to round a Span with calendar units, you need to
provide a relative datetime. By building rounding into the datetime
arithmetic routines, we absolve the caller of needing to manage this
carefully across two distinct operations. Since SignedDuration only
supports invariant units, there is no purpose to providing a relative
datetime and rounding is thus fully orthogonal. Moreover, if we
overloaded methods like Zoned::duration_since, then they would go from
infallible to fallible, which is kind of a bummer.

Closes #187