Rollup merge of #59374 - faern:simplify-checked-duration-since, r=shepmaster

Simplify checked_duration_since

This follows the same design as we updated to in #56490. Internally, all the system specific time implementations are checked, no panics. Then the panicking publicly exported API can just call the checked version of itself and make do with a single panic (`expect`) at the top.

Since the internal sys implementations are now checked, this gets rid of the extra `if self >= &earlier` check in `checked_duration_since`. Except likely making the generated machine code simpler, it also reduces the algorithm from "Check panic condition -> call possibly panicking method" to just "call non panicking method".

Added two test cases:
* Edge case: Make sure `checked_duration_since` on two equal `Instant`s produce a zero duration, not a `None`.
* Most common/intended usage: Make sure `later.checked_duration_since(earlier)`, returns an expected value.

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