Optimize Vec's IntoIter's methods

[czipperz]

In particular nth and last. To do: implement nth_back.

[rust-highfive]

r? @bluss

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[varkor]

Have you benchmarked these changes?

[czipperz]

Related to #57245 (comment), I think this change would cause side effects in destructors to trigger in different orders and thus not be backwards compatible.

[czipperz]

To directly address your comment @varkor, these changes would take O(n) operations and reduce them to O(1) operations. But because IntoIter owns the element it loops over, it must do O(n) destructions in any case and by changing these methods in the given way, the elements will be dropped in a different order.