Implement .abs() for Duration

[abreis]

This PR adds a method to get the absolute (modulus) of a Duration.

It is useful for checking how much two DateTimes diverge without having to fall back to i64s.

[quodlibetor]

Thanks!

[ComputerDruid]

This seems like it doesn't handle nanos properly; -1s + .5s is -0.5s which should have an absolute value of .5s not 1.5s

[abreis]

You're correct: I didn't realize at the time that the nanos field always 'adds' to the secs field (which makes sense since you can't store -0 in the secs field).

[abreis]

This should do it:

    /// Returns the duration as an absolute (non-negative) value.
    #[inline]
    pub fn abs(&self) -> Duration {
        if self.secs < 0 && self.nanos != 0 {
            Duration { secs: (self.secs+1).abs(), nanos: NANOS_PER_SEC-self.nanos }
        } else {
            Duration { secs: self.secs.abs(), nanos: self.nanos }
        }
    }

[abreis]

I submitted #734.