der: refactor `Reader::read_nested` #1279

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	27s
test (1.75.0)	1m 30s
build (1.75.0, wasm32-unknown-unknown)	33s
test (stable)	1m 8s
build (stable, thumbv7em-none-eabi)	22s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	22s
	4m 22s

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