Expose double-ended range iteration

[charliermarsh]

Summary

Ranges::iter returns a mapped slice iterator that is already double-ended, but its opaque return type only exposes Iterator. Callers that need reverse borrowed traversal therefore have to materialize the segments.

This exposes DoubleEndedIterator in the return type, allowing callers to traverse borrowed range segments in either direction without cloning or allocating while preserving existing forward iteration.

This unblocks allocation-free descending range traversal in astral-sh/uv#20026.

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing charlie/expose-double-ended-range-iterator (606a945) with main (5f7dbc9)}