Convert any closure to async easily with `withCheckedContinuation`

Defer.swift

func foo(_ completion: (Bool) -> Void) {
    completion(Bool.random())
}

func bar() async -> Result<Bool, Never> {
    return await withCheckedContinuation { continuation in
        foo { result in
            continuation.resume(returning: .success(result))
        }
    }
}

func baz() {
    Task {
        guard case let .success(result) = await bar() else {
            return
        }

        // if the result is success, we unwrap the value and use it here
        if result {
            // All good!
        } else {
            // Oh no
        }
    }
}