Use repeat_n in Vec::extend_with

[DaniPopes]

Simplify Vec::extend_with to use extend(repeat_n(...)). This delegates to extend_trusted which essentially does the same thing but using a single loop; the "don't clone the last element" logic is handled inside of the repeat_n iterator.

This generates slightly better code by removing the manual store of the last element: https://godbolt.org/z/TG6GPY9Eh

[rustbot]

r? @Mark-Simulacrum

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