pkcs8

der: have `Reader::peek*` methods take `&mut self` #3031

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	52s
build (1.72.0, wasm32-unknown-unknown)	50s
build (stable, thumbv7em-none-eabi)	58s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	57s
test (1.72.0)	3m 30s
test (stable)	3m 29s
	10m 36s

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