Add `FromHex::as_hex_backwards` #242

Job	Run time
Test - 1.56.1 toolchain	3m 44s
Test - nightly toolchain	24s
Test - beta toolchain	22s
Test - stable toolchain	57s
Check for changes to the public API	1m 20s
Test 32-bit version	35s
Format - nightly toolchain	23s
Clippy - stable toolchain	30s
Cross test	1m 13s
	9m 28s

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