der: refactor `Reader::read_nested` #3089

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	35s
build (1.72.0, wasm32-unknown-unknown)	39s
minimal-versions	0s
test (1.72.0)	3m 27s
build (stable, thumbv7em-none-eabi)	28s
test (stable)	3m 30s
build (stable, wasm32-unknown-unknown)	1m 15s
	9m 54s

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