add DowncastIntoError::into_inner

[davidhewitt]

This PR adds DowncastIntoError::into_inner, as a way to get the Bound value back if the downcast fails.

The name into_inner was chosen to match std, which has several examples of this e.g. std::sync::PosionError, std::io::WriterPanicked.

Should solve the case in #3820 (comment)

[codspeed-hq]

CodSpeed Performance Report

Merging #3829 will improve performances by 12.82%

_{Comparing davidhewitt:downcast-into-error (aec8d5b) with main (1279467)}

Summary

⚡ 1 improvements
✅ 78 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:downcast-into-error	Change
⚡	not_a_list_via_downcast	244.4 ns	216.7 ns	+12.82%

[davidhewitt]

🙏 thanks for the quick approval!