implement UniqueEntitySlice #56812

Job	Run time
build (windows-latest)	11m 17s
ci	2m 54s
check-missing-examples-in-docs	32s
build (ubuntu-latest)	6m 15s
check-doc	10m 3s
build (macos-latest)	5m 26s
miri	12m 58s
run-examples-macos-metal	5m 59s
toml	1m 52s
typos	9s
check-bevy-internal-imports	7s
check-missing-features-in-docs	27s
markdownlint	1m 4s
check-compiles	5m 42s
check-compiles-no-std	1m 21s
msrv	57s
build-wasm-atomics	1m 32s
build-wasm	1m 1s
	1h 9m 36s

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