Align `fmt_hex_exact` and `Display` impls #427

Job	Run time
Cross test - stable toolchain	1m 5s
Format - nightly toolchain	13s
Test 32-bit version	26s
Docs - stable toolchain (recent)	9s
Test - MSRV toolchain (minimal)	2m 18s
Prepare	4s
Test - MSRV toolchain (recent)	2m 19s
Test - stable toolchain (minimal)	24s
Test - stable toolchain (recent)	23s
Docs - nightly toolchain (recent)	23s
Lint - nightly toolchain (recent)	24s
Test - nightly toolchain (minimal)	32s
Test - nightly toolchain (recent)	31s
	9m 11s

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