v2.1.0

Opt-in device section: user_request::device adds user::client (JSON key device), the device of the user's last sign-in from device-generator 1.0.2, drawn with the account's country, platform and locale at the day of the last login, with a documentation-range IP address when safe. Off by default: users without a device, their JSON, signatures and known answers are unchanged. usergen.hpp now includes devicegen.hpp, so device-generator must be on the include path (its data loads only on the first user with a device). Tested against address 2.1.0, birth 3.2.0, country 3.2.0, device 1.0.2, email 2.1.1, name 4.5.0, nickname 5.2.0 and phone 2.1.1. @dasmig/user 1.3.0 ships this combination.