tracing-subscriber: refactor timings

[joshka]

Motivation

This makes the fmt::Subscriber code a bit simpler, and preps the
Timing type for lifting elsewhere (e.g. to be made public and part
of a timing layer instead of hidden within this Subscriber) (see
#2946)

Solution

This commit refactors the Timings struct to use Duration instead of
u64 to store the idle and busy times of a span, and moves the logic to
update the timings to the Timings struct itself. This commit also
introduces a display method to the Timings struct that returns the
idle and busy times as HumanReadableDuration instances (renamed from
TimingDisplay).

Replaces: #2944

[mladedav]

I like this more than #2944 👍

[joshka]

What about renaming this to SpanTimings instead of Timings? It's currently private, so this isn't breaking.

Side note:
My personal style would be to move this to its own small module, as it's a neatly self contained piece that doesn't depend on other parts of the library. I like using modules to reduce the cognitive load of reading larger source files, but I don't know how this squares with the library maintainers, and everyone has different preferences on what sizes matter.