* refactor: move xml_db into format::xml_db

* refactor: move database types to db::types

* refactor: split yubikey functionality out into key::yubikey

* refactor: move variant_dictionary and hmac_block_stream into format

* refactor: move io to format

* refactor: further split files by functionality

* several impl Database for opening, saving, merging
* move attachment, autotype, color, custom_data, history, icon, tiems
  into separate modules under db::types

* refactor: move merge-related Group functions to db::merge

* refactor: move totp functions for entry into db::otp