Implement address book for ENS/custom names

[bitgamma]

Allow user to store address => name couples on Shell. This address book needs to be both encrypted and signed using the card to guarantee privacy and security.

Since the db is custom we do not need the name to be necessarily a registered ENS name, it could be any name and it could be applied to bitcoin as well.

To check:

• Is it more efficient to store each record as a separate file system entry or should it be monolithic
• How large is an entry, and how much memory we should allocate for this