Add `FromHex::as_hex_backwards` #244

Job	Run time
Test - 1.56.1 toolchain	3m 30s
Test - nightly toolchain	22s
Test - beta toolchain	18s
Test - stable toolchain	51s
Check for changes to the public API	1m 17s
Test 32-bit version	29s
Format - nightly toolchain	21s
Clippy - stable toolchain	26s
Cross test	1m 17s
	8m 51s

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