We check if a currencies are in a list ([example][0]). We can speed this
up, and better clarify intent, if we use a `Set` instead of an array.
[0]:
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|---|---|---|
| .. | ||
| CachedBadge.swift | ||
| DonationUtilities.swift | ||
| Stripe.swift | ||
| SubscriptionManager.swift | ||
| SubscriptionReceiptCredentialRedemptionJob.swift | ||