Auto merge of #5597 - esamudera:slice_iter_next, r=flip1995

Auto merge of #5597 - esamudera:slice_iter_next, r=flip1995

New lint: iter_next_slice

Hello, this is a work-in-progress PR for issue: https://github.com/rust-lang/rust-clippy/issues/5572

I have implemented lint to replace `iter().next()` for `slice[index..]` and `array` with `get(index)` and `get(0)` respectively. However since I made a lot of changes, I would like to request some feedback before continuing so that I could fix mistakes.

Thank you!

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changelog: implement `iter_next_slice` lint and test, and modify `needless_continues`, `for_loop_over_options_result` UI tests since they have `iter().next()`