Span: Ergonomics Improvements

[martintmk]

First, let me say that I love the library. It just makes sense and the APIs are great to work with. Here are my thoughts after playing with the Span type:

Easy conversion into the Duration type:

It's a very common for service to get a span by subtracting timestamps. For example, by parsing retry-after header and applying sleep operation. Usually, sleep operation works with Duration and ti would be nice to have a built-in support for converting Span into Duration:

let span = 1.minute();
let duration: Duration = span.try_into().unwrap();
assert_eq!(duration, Duration::from_secs(60));

Check whether the Span is positive

Currently, there is is_zero and is_negative APIs on the Span, I miss the is_positive method that tells me the Span contains som non-zero positive value. Usage would be:

let diff = timestamp_2 - timestamp_1;

if diff.is_positive() {
    thread::sleep(diff.try_into());
} 

I was also missing saturating_add and saturating_sub, but that's covered by #10.

[BurntSushi]

For Duration, I think that's #21. Although it's important to note that spans with units greater than days require a reference date. So you won't be able to just call Duration::try_from(span) because it doesn't have enough information. With that said, we could provide a TryFrom<Span> for Duration that just fails if it has any units above days. I'm not 100% certain that's an OK API to add, but I think it probably seems fine.

One approach that can be done today is to feed the output of Span::total into Duration::try_from_secs_f64. That will work even with calendar units since Span::total requires a reference date for spans with non-zero units greater than days.

Now, going from Duration to Span is much easier, or would be easier if Duration were signed. Otherwise it would just be Span::new().seconds(d.as_secs()).nanoseconds(d.subsec_nanos()). So a TryFrom<Duration> for Span would be very nice to have. (It could almost be infallible except for the fact that a Duration could result in overflow of a Span.)

I'm definitely fine with adding is_positive.

[BurntSushi]

See: #21 (comment)