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der: `PemReader` simplifications #3039

Job	Run time
minimal-versions	0s
build (1.72.0, thumbv7em-none-eabi)	33s
test (1.72.0)	1m 13s
build (1.72.0, wasm32-unknown-unknown)	34s
test (stable)	1m 4s
build (stable, thumbv7em-none-eabi)	26s
build (stable, wasm32-unknown-unknown)	33s
	4m 23s

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