Merge rust-bitcoin/hex-conservative#122: Fix `HexToBytesIter::size_hint` #1

Job	Run time
Cross test - stable toolchain	1m 8s
Docs - stable toolchain (recent)	15s
Format - nightly toolchain	14s
Test 32-bit version	51s
Test - MSRV toolchain (minimal)	1m 36s
Prepare	2s
Test - stable toolchain (minimal)	31s
Test - stable toolchain (recent)	28s
Test - MSRV toolchain (recent)	1m 33s
Docs - nightly toolchain (recent)	30s
Test - nightly toolchain (minimal)	40s
Lint - nightly toolchain (recent)	35s
Test - nightly toolchain (recent)	34s
	8m 57s

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