der: refactor `Reader::read_nested` #892

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	26s
minimal-versions	0s
build (1.75.0, wasm32-unknown-unknown)	34s
test (1.75.0)	24s
build (stable, thumbv7em-none-eabi)	28s
test (stable)	17s
build (stable, wasm32-unknown-unknown)	28s
	2m 37s

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