Check

Implement the `Subscription` type #124

Job	Run time
generate-matrices	31s
lint	47s
SPM (Xcode 15.3)	1m 1s
Example app, macOS (Xcode 15.3, Swift 5)	3m 3s
SPM (Xcode 16-beta)	1m 1s
Xcode, macOS (Xcode 15.3)	1m 57s
Example app, iOS (Xcode 15.3, Swift 5)	2m 5s
Xcode, iOS (Xcode 15.3)	2m 21s
Example app, tvOS (Xcode 15.3, Swift 5)	2m 28s
Xcode, tvOS (Xcode 15.3)	2m 47s
Example app, macOS (Xcode 16-beta, Swift 6)	3m 57s
Xcode, macOS (Xcode 16-beta)	1m 45s
Example app, iOS (Xcode 16-beta, Swift 6)	3m 25s
Xcode, iOS (Xcode 16-beta)	2m 32s
Example app, tvOS (Xcode 16-beta, Swift 6)	2m 11s
Xcode, tvOS (Xcode 16-beta)	1m 51s
all-checks-completed	0s
	33m 42s

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