der: refactor `Reader::read_nested` #3136

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	26s
minimal-versions	0s
test (1.72.0)	34s
build (1.72.0, wasm32-unknown-unknown)	24s
test (stable)	51s
build (stable, thumbv7em-none-eabi)	36s
build (stable, wasm32-unknown-unknown)	16s
	3m 7s

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