Rollup merge of #146410 - hkBst:repeat-1, r=jhpratt

Iterator repeat: no infinite loop for `last` and `count`

This removes two cases of infinite looping from [`Repeat`](https://doc.rust-lang.org/nightly/std/iter/struct.Repeat.html):
- [`last`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.last): By viewing the iterator as returning None after [omega](https://en.wikipedia.org/wiki/Ordinal_number) calls to `next`, this method can simply return the repeated element.

- [`count`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.count): From its docs: """The method does no guarding against overflows, so counting elements of an iterator with more than [usize::MAX](https://doc.rust-lang.org/nightly/std/primitive.usize.html#associatedconstant.MAX) elements either produces the wrong result or panics. If overflow checks are enabled, a panic is guaranteed.""", so a panic'ing impl is allowed by the docs, and is more honest than an infinite loop.

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