der: refactor `Reader::read_nested` #1865

Job	Run time
build (1.75.0, thumbv7em-none-eabi)	1m 4s
build (1.75.0, wasm32-unknown-unknown)	1m 2s
build (stable, thumbv7em-none-eabi)	55s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	47s
test (1.75.0)	4m 53s
test (stable)	4m 46s
fuzz	2m 15s
	15m 42s

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