Suggestion: Replace `d.subsec_nanos() / 1_000_000` with `d.subsec_millis()`

[kennytm]

rust-lang/rust#46507 may soon be stabilized. Code that computes the milliseconds/microseconds-part of a duration could use the .subsec_millis()/.subsec_micros() methods directly instead of going through a computation with lots of zeros. Clippy should add a lint for the following patterns:

Old	New
dur.subsec_nanos() / 1_000_000	dur.subsec_millis()
dur.subsec_nanos() / 1_000	dur.subsec_micros()

This mainly helps modernizing existing code.