Add contains convenience impl for Backend #468

Job	Run time
docs	1m 33s
macos / Test macOS	1m 37s
ci / Build	5m 17s
windows / Test Windows	3m 5s
ci / Lint	1m 6s
ci / Test (0)	7m 21s
ci / Test (1)	6m 32s
ci / Test (2)	14m 27s
ci / Test (3)	7m 7s
ci / Test (4)	10m 8s
ci / Test (5)	5m 36s
ci / Test (6)	11m 36s
ci / Test (7)	9m 13s
ci / Test (8)	5m 21s
ci / Test (9)	6m 54s
ci / Publish Unit Tests Results	1s
	1h 36m 54s

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