spki

der: refactor `Reader::read_nested` #3116

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	27s
build (1.72.0, wasm32-unknown-unknown)	27s
test (1.72.0)	1m 22s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	1m 8s
build (stable, wasm32-unknown-unknown)	26s
minimal-versions	0s
	4m 10s

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