der: have `Reader::peek*` methods take `&mut self` #759

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	30s
build (1.75.0, wasm32-unknown-unknown)	31s
test (1.75.0)	30s
build (stable, thumbv7em-none-eabi)	26s
build (stable, wasm32-unknown-unknown)	20s
test (stable)	18s
minimal-versions	0s
	2m 35s

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