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der: `PemReader` simplifications #3041

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	34s
minimal-versions	0s
test (1.72.0)	1m 18s
build (1.72.0, wasm32-unknown-unknown)	32s
build (stable, thumbv7em-none-eabi)	35s
build (stable, wasm32-unknown-unknown)	27s
test (stable)	1m 8s
	4m 34s

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