Combine `QrCodeEvent`, `SasEvent` and `VerificationEvent` (#3386)
* Move IReciprocateQr to `crypto-api` and rename
* Move ISasEvent to `crypto-api`, and rename
... and add some ✨comments✨
* Combine QrCodeEvent, SasEvent and VerificationEvent together
... as a precursor to extracting a single `Verifier` interface for `SAS` and `ReciprocateQRCode`.
`enum`s are slightly magical things that have both a type and a value, so we
have to re-export their backwards-compatibility fudges twice.
* Update src/crypto/verification/Base.ts