move Cow[u8] conversions into conversions::std::slice module

[davidhewitt]

Just moving code around.

A follow-up to #3784 (comment)

[codspeed-hq]

CodSpeed Performance Report

Merging #3799 will improve performances by 20.41%

_{Comparing davidhewitt:move-cow (7281268) with main (c9c6f92)}

Summary

⚡ 3 improvements
✅ 76 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:move-cow	Change
⚡	mapping_from_dict	327.8 ns	272.2 ns	+20.41%
⚡	not_a_list_via_downcast	242.8 ns	215 ns	+12.92%
⚡	sequence_from_list	355.6 ns	300 ns	+18.52%