der

der: add `SetOfRef` struct #4521

Job	Run time
build-no-std	2m 26s
test-cross (powerpc-unknown-linux-gnu)	2m 12s
test-careful	1m 28s
test-cross (armv7-unknown-linux-gnueabi)	2m 8s
test-derive (stable)	33s
test-wasm	54s
test-derive (1.85.0)	32s
test-heapless	38s
test-miri (s390x-unknown-linux-gnu)	1m 8s
test-miri (x86_64-unknown-linux-gnu)	1m 0s
minimal-versions / minimal-versions	2m 55s
test (aarch64-apple-darwin, stable, macos-latest)	7m 56s
test (aarch64-apple-darwin, 1.85.0, macos-latest)	8m 26s
test (x86_64-pc-windows-msvc, stable, windows-latest)	14m 40s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	7m 45s
test (x86_64-unknown-linux-gnu, 1.85.0)	8m 30s
test (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	7m 50s
test (aarch64-unknown-linux-gnu, stable, ubuntu-24.04-arm)	6m 42s
test (x86_64-pc-windows-msvc, 1.85.0, windows-latest)	16m 12s
test (aarch64-unknown-linux-gnu, 1.85.0, ubuntu-24.04-arm)	9m 50s
test (x86_64-unknown-linux-gnu, stable)	7m 7s
	1h 50m 52s

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