std: implement clear via truncate

[joboet]

This gets rid of some unsafe. truncate(0) is even documented to be equivalent to clear, this makes that equivalence even more obvious.

[rustbot]

r? @Mark-Simulacrum

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[xtqqczze]

clear was intentionally implemented this way (rather than using truncate(0)) for performance reasons, as introduced in 9c59d04.

cc: @nnethercote

[Mark-Simulacrum]

r? @nnethercote

[nnethercote]

I just wrote a microbenchmark and the difference between the two is still measurable. So I'd prefer to keep the code as is. It would be useful to add a comment explaining why truncate(0) is not used, if you are happy to do that? (I should have done that in #96002.)