rework extract_argument code to populate holders as intermediate values

[davidhewitt]

This is an experiment to use type inference & "holders" to change the extract_argument code to no longer need to take &mut holder inputs to argument extraction but instead produce holders as outputs from extraction. The holders then have a subsequent "unpack" phase.

I think this might enable error messages for unimplemented PyFunctionArgument to look a bit nicer, I'm going to play with that shortly. Pushing now to see what the pipeline thinks.

Will update this PR with a more detailed description as the code takes shape...

[codspeed-hq]

Merging this PR will not alter performance

✅ 105 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing davidhewitt:extract-rework (b3368d0) with main (86c48d3)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩