Make SignedDeviceKeys use Signatures, instead of just a HashMap
On writing the structs for serialising one-time-keys I used a different approach from the structs for the device keys:
I abstracted the
HashMap for device signatures into its own
This generally improves readability and also provides a generic approach to expressing signatures for these structs. As many JSON objects need to contain a "signatures" key, having such an approach would greatly reduce code duplication.
Efforts should be aligned with #1 (closed)