Merge rust-bitcoin/hex-conservative#127: Align `fmt_hex_exact` and `D… #431

Job	Run time
Cross test - stable toolchain	1m 4s
Docs - stable toolchain (recent)	12s
Format - nightly toolchain	11s
Test 32-bit version	26s
Test - MSRV toolchain (minimal)	2m 22s
Prepare	2s
Test - stable toolchain (minimal)	23s
Test - MSRV toolchain (recent)	2m 21s
Test - stable toolchain (recent)	22s
Docs - nightly toolchain (recent)	26s
Lint - nightly toolchain (recent)	25s
Test - nightly toolchain (minimal)	27s
Test - nightly toolchain (recent)	30s
	9m 11s

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