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der: refactor `Header::peek` #3054

Job	Run time
minimal-versions	0s
build (1.72.0, thumbv7em-none-eabi)	35s
build (1.72.0, wasm32-unknown-unknown)	34s
test (1.72.0)	1m 15s
build (stable, thumbv7em-none-eabi)	51s
test (stable)	1m 8s
build (stable, wasm32-unknown-unknown)	29s
	4m 52s

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