add dedicated parsing and printing routines for `std::time::Duration`, along with `jiff::fmt::serde` helpers

[LukeMathWalker]

I've migrated Pavex, the web framework I'm working on, to jiff in this commit.
Things went pretty smoothly, the documentation was excellent all around.

There was only one instance that had me thinking "this could be better": this hunk. Graceful shutdown timeouts, time-to-live for sessions—these are all values that must be modelled as positive durations. I have therefore relied on std::time::Duration rather than jiff's SignedDuration.
Nonetheless, I want to give users the possibility to pass these configuration values using a human-readable format (e.g. 2 minutes), a job I used to delegate to the humantime crate.

I wonder: would it possible for jiff to add facilities to parse std::time::Duration in the same way it currently parses SignedDuration?
I'm naively assuming that:

• Most of the parsing logic would be identical, sign aside
• It would show up as an additional module (e.g. std_duration?) under jiff::fmt::serde.

[BurntSushi]

Yeah I saw that PR and that speed-bump and was thinking the same thing. I think this is reasonable.

I think we'll want to add dedicated methods for dealing with std::time::Duration on jiff::fmt::friendly::{SpanParser, SpanPrinter} as well. This might require some hoops to jump through. Today, we rely on just parsing a SignedDuration first, which means anything greater than SignedDuration::MAX but less than std::time::Duration::MAX is impossible to parse. That's a little harder to justify when we have dedicated methods for std::time::Duration. I don't have a good sense of how hard it will be to refactor the existing code for this. In the worst case, we duplicate it. (A lot of it is already duplicated for Span and SignedDuration already, IIRC.)

And yeah, I think adding serde helpers to jiff::fmt::serde makes sense to. I like unsigned_duration better since it's more descriptive, although it is longer. And we'll need parsing helpers there too, unlike for jiff::SignedDuration.

One thing that is harder to do is nice integration with std::fmt::Display and std::str::FromStr. The only way that works is if we new-type std::time::Duration... But I'm not sure that's worth it if we can do the above.

[BurntSushi]

I added the "help wanted" label here because I feel like the resolution to this issue has a somewhat clear path. So if someone wanted to work on this, I think I could mentor it. If someone wants to do this, feel free to post questions here or ping me on Discord (handle is burntsushi5).

I will do this eventually myself though otherwise!