[Contacts rewrite] Separate mapping from vCard generation/parsing

[rfc2822]

• Contact should only be a data class (as a first step, before it is completely removed) → remove Contact.fromReader and contact.writeVCard(), should be replaced by calling mapping + vCard handling separately
• ContactReader to ContactWriter are then only mappers and can be named accordingly
  • ContactReader (vcf → Contact) becomes ContactBuilder (builds Contact and later Entity)
  • ContactWriter (Contact → vcf) becomes ContactHandler (processes Contact and later Entity)
• ContactProcessor (centralized mapping) is absorbed by the new ContactBuilder and ContactHandler