spki

der: refactor `Header::peek` #3127

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	31s
minimal-versions	0s
test (1.72.0)	1m 16s
build (1.72.0, wasm32-unknown-unknown)	40s
test (stable)	1m 5s
build (stable, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	19s
	4m 11s

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