der

der: add `SetOfRef` struct #4525

Job	Run time
test-derive (stable)	32s
test-cross (powerpc-unknown-linux-gnu)	2m 35s
test-careful	1m 19s
test-cross (armv7-unknown-linux-gnueabi)	2m 14s
build-no-std	2m 34s
test-wasm	59s
test-heapless	39s
test-derive (1.85.0)	29s
test-miri (s390x-unknown-linux-gnu)	1m 13s
test (aarch64-apple-darwin, stable, macos-latest)	6m 28s
test-miri (x86_64-unknown-linux-gnu)	59s
minimal-versions / minimal-versions	3m 3s
test (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	7m 45s
test (aarch64-unknown-linux-gnu, stable, ubuntu-24.04-arm)	6m 55s
test (aarch64-unknown-linux-gnu, 1.85.0, ubuntu-24.04-arm)	10m 9s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	7m 24s
test (x86_64-unknown-linux-gnu, stable)	6m 59s
test (x86_64-pc-windows-msvc, 1.85.0, windows-latest)	17m 6s
test (x86_64-unknown-linux-gnu, 1.85.0)	8m 40s
test (aarch64-apple-darwin, 1.85.0, macos-latest)	9m 25s
test (x86_64-pc-windows-msvc, stable, windows-latest)	15m 3s
	1h 52m 30s

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