Add SystemTime::{MIN, MAX}

[cvengler]

Accepted ACP: rust-lang/libs-team#692
Tracking Issue: #149067

---

This merge request introduces two new constants to SystemTime: MIN and MAX, whose values represent the maximum values for the respective data type, depending upon the platform.

Technically, this value is already obtainable during runtime with the following algorithm:
Use SystemTime::UNIX_EPOCH and call checked_add (or checked_sub) repeatedly with Duration::new(0, 1) on it, until it returns None.
Mathematically speaking, this algorithm will terminate after a finite amount of steps, yet it is impractical to run it, as it takes practically forever.

Besides, this commit also adds a unit test to verify those values represent the respective minimum and maximum, by letting a checked_add and checked_sub on it fail.

In the future, the hope of the authors lies within the creation of a SystemTime::saturating_add and SystemTime::saturating_sub, similar to the functions already present in std::time::Duration.
However, for those, these constants are crucially required, thereby this should be seen as the initial step towards this direction.
With this change, implementing these functions oneself outside the standard library becomes feasible in a portable manner for the first time.

This feature (and a related saturating version of checked_{add, sub} has been requested multiple times over the course of the past few years, most notably:

• Subtracting Duration from Instant on Mac is panic-prone #100141
• Deprecate std::time::Instant::saturating_duration_since()? #133525
• Cannot subtract dates before UNIX_EPOCH when targeting wasm32 #105762
• Feature Request: saturating_add(Duration) for SystemTime / Instant #71224
• Document arithmetic operations on Instant #45448
• resolve: Some renaming, refactoring and comments #52555