Rollup merge of #114238 - jhpratt:fix-duration-div, r=thomcc

Fix implementation of `Duration::checked_div`

I ran across this while running some sanity checks on `time`. Quickcheck immediately found a bug, and as I'd modified the code from `std` I knew there was a bug here as well.

tl;dr this code fails ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1189a3efcdfc192c27d6d87815359353))

```rust
use std::time::Duration;

fn main() {
    assert_eq!(
        Duration::new(1, 1).checked_div(7),
        Some(Duration::new(0, 142_857_143)),
    );
}
```

The existing code determines that 1/7 = 0 (seconds), 1/7 = 0 (nanoseconds), 1 billion / 7 = 142,857,142 (extra nanoseconds). The billion comes from multiplying the remainder of the seconds (1) by the number of nanoseconds in a second. However, **this wrongly ignores any remaining nanoseconds**. This PR takes that into consideration, adds a test, and also changes the roundabout way of calculating the remainder into directly computing it.

Note: This is _not_ a rounding error. This result divides evenly.

`@rustbot` label +A-time +C-bug +S-waiting-on-reviewer +T-libs

0 files changed, 0 insertions, 0 deletions