Rollup merge of #103446 - the8472:tra-array-chunks, r=Mark-Simulacrum

Specialize `iter::ArrayChunks::fold` for TrustedRandomAccess iterators

```
OLD:
test iter::bench_trusted_random_access_chunks                      ... bench:         368 ns/iter (+/- 4)
NEW:
test iter::bench_trusted_random_access_chunks                      ... bench:          30 ns/iter (+/- 0)
```

The resulting assembly is similar to #103166 but the specialization kicks in under different (partially overlapping) conditions compared to that PR. They're complementary.

In principle a TRA-based specialization could be applied to all `ArrayChunks` methods, including `next()` as we do for `Zip` but that would have all the same hazards as the Zip specialization. Only doing it for `fold` is far less hazardous. The downside is that it only helps with internal, exhaustive iteration. I.e. `for _ in` or `try_fold` will not benefit.

Note that the regular, `try_fold`-based and the specialized `fold()` impl have observably slightly different behavior. Namely the specialized variant does not fetch the remainder elements from the underlying iterator. We do have a few other places in the standard library where beyond-the-end-of-iteration side-effects are being elided under some circumstances but not others.

Inspired by https://old.reddit.com/r/rust/comments/yaft60/zerocost_iterator_abstractionsnot_so_zerocost/

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