der: refactor `Reader::read_nested` (#1474) #2082

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 31s
test (1.65.0)	2m 39s
build (1.65.0, wasm32-unknown-unknown)	2m 25s
test (stable)	19s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	12s
minimal-versions / minimal-versions	23s
	8m 42s

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