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 Signatures struct.

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)