Auto merge of #50364 - LukasKalbertodt:improve-duration-debug-impl, r=KodrAus

Improve `Debug` impl of `time::Duration`

Hi there!

For a long time now, I was getting annoyed by the derived `Debug` impl of `Duration`. Usually, I use `Duration` to either do quick'n'dirty benchmarking or measuring the time of some operation in general. The output of the derived Debug impl is hard to parse for humans: is { secs: 0, nanos: 968360102 } or { secs: 0, nanos 98507324 } longer?

So after running into the annoyance several times (sometimes building my own function to print the Duration properly), I decided to tackle this. Now the output looks like this:

```
Duration::new(1, 0)                 => 1s
Duration::new(1, 1)                 => 1.000000001s
Duration::new(1, 300)               => 1.0000003s
Duration::new(1, 4000)              => 1.000004s
Duration::new(1, 600000)            => 1.0006s
Duration::new(1, 7000000)           => 1.007s
Duration::new(0, 0)                 => 0ns
Duration::new(0, 1)                 => 1ns
Duration::new(0, 300)               => 300ns
Duration::new(0, 4001)              => 4.001µs
Duration::new(0, 600300)            => 600.3µs
Duration::new(0, 7000000)           => 7ms
```

Note that I implemented the formatting manually and didn't use floats. No information is "lost" when printing. So `Duration::new(123_456_789_000, 900_000_001)` prints as `123456789000.900000001s`.

~~This is not yet finished~~, but I wanted to open the PR now already in order to get some feedback (maybe everyone likes the derived impl).

### Still ToDo:

- [x] Respect precision ~~and width~~ parameter of the formatter (see [this comment](https://github.com/rust-lang/rust/pull/50364#issuecomment-386107107))

### Alternatives/Decisions

- Should large durations displayed in minutes, hours, days, ...? For now, I decided not to because the current formatting is close the how a `Duration` is stored. From this new `Debug` output, you can still easily see what the values of `secs` and `nanos` are. A formatting like `3h 27m 12s 9ms` might be more appropriate for a `Display` impl?
- Should this rather be a `Display` impl and should `Debug` be derived? Maybe this formatting is too fancy for `Debug`? In my opinion it's not and, as already mentioned, from the current format one can still very easily determine the values for `secs` and `nanos`.
- Whitespace between the number and the unit?

### Notes for reviewers

- ~~The combined diff sucks. Rather review both commits individually.~~
- ~~In the unit test, I am building my own type implementing `fmt::Write` to test the output. Maybe there is already something like that which I can use?~~
- My `Debug` impl block is marked as `#[stable(...)]`... but that's fine since the derived Debug impl was stable already, right?

---

~~Apart from the main change, I moved all `time` unit tests into the `tests` directory. All other `libcore` tests are there, so I guess it was simply an oversight. Prior to this change, the `time` tests weren't run, so I guess this is kind of a bug fix. If my `Debug` impl is rejected, I can of course just send the fix as PR.~~ (this was already merged in #50466)

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