Add `FromHex::as_hex_backwards` #233

Job	Run time
Clippy - stable toolchain	16s
Test - stable toolchain	33s
Test - beta toolchain	21s
Check for changes to the public API	1m 17s
Test 32-bit version	26s
Format - nightly toolchain	21s
Test - nightly toolchain	22s
Cross test	1m 11s
Test - 1.56.1 toolchain	3m 34s
	8m 21s

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