add slice-ptr accessors to PyBuffer

[Person-93]

Add accessors to PyBuffer that get the data as a pointer to a slice. See #6267

[codspeed]

Merging this PR will improve performance by 13.23%

⚡ 1 improved benchmark
✅ 140 untouched benchmarks
⏩ 6 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_empty_class_init	32.4 µs	28.7 µs	+13.23%

Tip

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_{Comparing Person-93:buffer-slice-pointers (96b31c5) with main (da9da49)}

Footnotes

1. 6 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. ↩

[davidhewitt]

Thanks

[davidhewitt]

Would Option<NonNull<[T]>> be better? (And similar on the other methods.)

[davidhewitt]

Thanks, this looks fine to me now.

I think it might be worth adding some documentation explaining to users that they are responsible for avoiding data races while reading / writing the buffer. We can do that as part of #6381