We still need the AxolotlKit model classes to migrate old sessions,
but we don't need any of the actual protocol support. This also
means we can drop HKDFKit.
Additionally, we do still use some utilities from AxolotlKit:
- AxolotlExceptions.h: NSException names, should eventually be
replaced by NSErrors everywhere
- NSData+keyVersionByte.h: prepend/remove public key type byte,
should eventually be replaced by strong types (ECPublicKey)
- SPKProtocolContext.h: defines the SPKProtocol{Read,Write}Context
marker protocols, should be replaced by direct use of
SDSAny{Read,Write}Transaction
