der: refactor `Reader::read_nested` #3118

Job	Run time
build (1.72.0, thumbv7em-none-eabi)	29s
minimal-versions	0s
build (1.72.0, wasm32-unknown-unknown)	32s
test (1.72.0)	1m 9s
build (stable, thumbv7em-none-eabi)	20s
test (stable)	1m 10s
build (stable, wasm32-unknown-unknown)	33s
	4m 13s

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