Auto merge of #48945 - clarcharr:iter_exhaust, r=Kimundi

Auto merge of #48945 - clarcharr:iter_exhaust, r=Kimundi

Replace manual iterator exhaust with for_each(drop)

This originally added a dedicated method, `Iterator::exhaust`, and has since been replaced with `for_each(drop)`, which is more idiomatic.

<del>This is just shorthand for `for _ in &mut self {}` or `while let Some(_) = self.next() {}`. This states the intent a lot more clearly than the identical code: run the iterator to completion.

<del>At least personally, my eyes tend to gloss over `for _ in &mut self {}` without fully paying attention to what it does; having a `Drop` implementation akin to:

<del>`for _ in &mut self {}; unsafe { free(self.ptr); }`</del>

<del>Is not as clear as:

<del>`self.exhaust(); unsafe { free(self.ptr); }`

<del>Additionally, I've seen debate over whether `while let Some(_) = self.next() {}` or `for _ in &mut self {}` is more clear, whereas `self.exhaust()` is clearer than both.