Add real async iterator for ByteStreams

[akx]

Summary

Previously, __aiter__ was implemented as a generator function, and as such could get garbage collected when not completely exhausted in a way that has recent versions of trio complaining.

Using a real, if trivial, object to do this causes less complaints.

Port of my PR encode/httpx#3777

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
  • No need to.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing akx:trio-bytestream-timeout-hx2 (1b58c6e) with main (82b9e2d)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}