der: refactor `Reader::read_nested` #1220

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	29s
build (1.75.0, wasm32-unknown-unknown)	32s
minimal-versions	0s
build (stable, thumbv7em-none-eabi)	29s
build (stable, wasm32-unknown-unknown)	25s
test (1.75.0)	31s
test (stable)	38s
	3m 4s

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