Auto merge of #56932 - clarcharr:iter_refactor, r=Centril

Refactor core::iter module

A while back, I refactored `core::ops` in #42523 because the module had become a giant mess and was difficult to modify. Now, I'm doing the same with the `core::iter` module.

Like the `core::ops` refactor, things have been split up into multiple commits to make rebasing easier, and so that you can follow changes. Although the diffs are hard to decipher, the only actual code changes I've made in the first few commits are to modify exports and imports. I save all of the actual code refactoring, e.g. modifying what methods are called, for the end.

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