Rename Iterator::array_chunks to Iterator::chunks

[rossmacarthur]

The most logical way to yield chunks from an iterator is with arrays. Originally I named this to align with slice::array_chunks but I think chunks makes more sense for iterators and it aligns with the next_chunk method.

Related #100450

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[jdahlstrom]

I would expect chunks to be reserved for a possible future method that takes a runtime size and returns an iterator over subiterators. Ie. what Itertools::chunks does.

[rossmacarthur]

Yeah, good point 🤔, I wonder if next_chunk is better called next_array