switch `impl From<Timestamp> for std::time::SystemTime` to `TryFrom`

[BurntSushi]

Specifically, here is the current impl:

jiff/src/timestamp.rs

Lines 2815 to 2831 in 42d83ff

	#[cfg(feature = "std")]
	impl From<Timestamp> for std::time::SystemTime {
	#[inline]
	fn from(time: Timestamp) -> std::time::SystemTime {
	let unix_epoch = std::time::SystemTime::UNIX_EPOCH;
	let sdur = time.as_duration();
	let dur = sdur.unsigned_abs();
	// These are guaranteed to succeed because we assume that SystemTime
	// uses at least 64 bits for the time, and our durations are capped via
	// the range on UnixSeconds.
	if sdur.is_negative() {
	unix_epoch.checked_sub(dur).expect("duration too big (negative)")
	} else {
	unix_epoch.checked_add(dur).expect("duration too big (positive)")
	}
	}
	}

It specifically notes that we assume a SystemTime always uses 64 bits. But this is not a documented guarantee. And indeed, it turns out it's not always true: rust-lang/rust#150505

This is a breaking change, so it will need to be part of the Jiff 1.0 release.