update coverage to use `cargo llvm-cov`

[davidhewitt]

Attempt to simplify things a bit.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #109 will improve performances by 10.56%

_{Comparing llvm-cov (19f2e86) with main (366d16a)}

Summary

⚡ 1 improvements ✅ 72 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	llvm-cov	Change
⚡	python_parse_string_array_not_cached	40.5 µs	36.6 µs	+10.56%

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:loudspeaker: Thoughts on this report? Let us know!

[davidhewitt]

Looks like more faff than I expected, will park for today.