Duration::ZERO composes better with match and various other things,
at the cost of an occasional parens, and results in less work for the
optimizer, so let's use that instead.
This expands time's test suite to use more and in more places the
range of methods and constants added to Duration in recent
proposals for the sake of testing more API surface area and
improving legibility.
Make the following methods of `Duration` unstable const under `duration_const_2`:
- `from_secs_f64`
- `from_secs_f32`
- `mul_f64`
- `mul_f32`
- `div_f64`
- `div_f32`
This results in all methods of `Duration` being (unstable) const.
Also adds tests for these methods in a const context, moved the test to `library` as part of #76268.
Possible because of #72449, which made the relevant `f32` and `f64` methods const.
Tracking issue: #72440
Ole Bertram