Define return types of SubtleCrypto methods with more specific types

[saschanaz]

Currently most of them use just Promise<any>, but many algorithms always returns the same type of value:

• encrypt: ArrayBuffer
• decrypt: ArrayBuffer
• sign: ArrayBuffer
• verify: boolean
• generateKey: (CryptoKey or CryptoKeyPair)
• etc.

I guess it's any for extensibility but I doubt the above methods will have different return values, or can they?

[twiss]

Hey 👋 I think the rationale is/was that theoretically, another spec could define an algorithm (as per section 3.1) with operations which return a different type. That being said, I think that would obviously be a bad idea, and we may want to get rid of this extensibility anyway (as per #286 (comment)). After we get rid of that, we could make these types more specific, too 👍