der

der: have `Reader::peek*` methods take `&mut self` #2862

Job	Run time
minimal-versions	0s
build (1.71.0, thumbv7em-none-eabi)	1m 44s
derive (1.71.0)	27s
build (1.71.0, wasm32-unknown-unknown)	1m 44s
derive (stable)	20s
build (stable, thumbv7em-none-eabi)	1m 25s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	10m 49s
build (stable, wasm32-unknown-unknown)	1m 33s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	10m 8s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	12m 37s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	10m 24s
	51m 11s

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