Rollup merge of #72368 - CAD97:rangeto, r=dtolnay

Resolve overflow behavior for RangeFrom

This specifies a documented unspecified implementation detail of `RangeFrom` and makes it consistently implement the specified behavior.

Specifically, `(u8::MAX).next()` is defined to cause an overflow, and resolve that overflow in the same manner as the `Step::forward` implementation.

The inconsistency that has existed is `<RangeFrom as Iterator>::nth`. The existing behavior should be plain to see after #69659: the skipping part previously always panicked if it caused an overflow, but the final step (to set up the state for further iteration) has always been debug-checked.

The inconsistency, then, is that `RangeFrom::nth` does not implement the same behavior as the naive (and default) implementation of just calling `next` multiple times. This PR aligns `RangeFrom::nth` to have identical behavior to the naive implementation. It also lines up with the standard behavior of primitive math in Rust everywhere else in the language: debug checked overflow.

cc @Amanieu

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Followup to #69659. Closes #25708 (by documenting the panic as intended).

The documentation wording is preliminary and can probably be improved.

This will probably need an FCP, as it changes observable stable behavior.

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