der

der: refactor `Reader::read_nested` #2989

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	1m 30s
derive (1.71.0)	28s
build (1.71.0, wasm32-unknown-unknown)	1m 23s
minimal-versions	0s
derive (stable)	19s
build (stable, thumbv7em-none-eabi)	1m 36s
test (i686-unknown-linux-gnu, ubuntu-latest, 1.71.0, sudo apt update && sudo apt install gcc-mult...	10m 56s
build (stable, wasm32-unknown-unknown)	1m 29s
test (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mult...	9m 57s
test (x86_64-unknown-linux-gnu, ubuntu-latest, 1.71.0)	10m 16s
test (x86_64-unknown-linux-gnu, ubuntu-latest, stable)	10m 1s
	47m 55s

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