Various methods and trait impls for time arithmetic

[gefjon]

Description of Changes

This commit adds checked_add and checked_sub methods for arithmetic with Timestamp and TimeDuration, and implementations of Add, Sub, AddAssign and SubAssign. Prior to this, BitCraft's code had to do convoluted conversions from Timestamp into SystemTime, then do arithmetic, then convert back to TimeStamp.

Specifically, we add methods and trait impls for T x U -> V, where x is a binary operator, either + or -:

• Timestamp x TimeDuration -> Timestamp
• Timestamp x Duration -> Timestamp.
  • This one is kind of weird, since we convert the Duration into a TimeDuration before doing math with it, which may be a lossy conversion depending on platform.
• TimeDuration x TimeDuration -> TimeDuration.

These correspond to the operations that Rust exposes on std::time::SystemTime and std::time::Duration, plus allowing Duration as a convenience.

I have chosen not to implement
TimeDuration x Duration or Duration x TimeDuration operations, under the theory that if you're already working with a TimeDuration, you don't need to be shielded from the conversion.

Closes #2367 .

API and ABI breaking changes

This PR adds new APIs which will become stabilized. It does not remove or change any existing APIs.

Expected complexity level and risk

2? Some minor complexity in sats::Timestamp x std::time::Duration arithmetic.

Testing

• Wrote a couple of simple sanity-check proptests.

[gefjon]

Presumably we should do something similar for C#.

[gefjon]

It occurs to me that we could also provide impl Sub<Timestamp> for Timestamp { type Output = TimeDuration; }. This would have the same behavior as Timestamp::time_duration_since. Rust doesn't provide this on SystemTime because their Duration is unsigned and so such a method would frequently panic, but we have signed TimeDuration so our version would only panic on under/overflow.

[kazimuth]

This makes sense. If you wouldn't mind making the corresponding change at the same time in C# that would be nice, it's in bindings-csharp/BSATN.Runtime/Builtins.rs, there are already a few operator overrides in there.

I'm wondering if we should put the corresponding operations inside https://stackoverflow.com/questions/2056445/no-overflow-exception-for-int-in-c blocks in C#. I think it would make sense for the C# API to always have exceptions thrown on overflow. Checked blocks are enforced syntactically, so the user has no other way to enable checking for time operations on the C# side.

[coolreader18]

LGTM