0.22.0

• A within method has been added to the reader to allow iterating
  over all records in the database. Pull request by Ross McFarland.
  Github #50.
• Database structs in maxminddb::geoip2 have been updated. Most
  noticeably, an Enterprise struct has been added and the model
  module has been replaced by city and country modules. Also,
  several missing fields have been added.
• Mmap is now re-exported for convenience. Pull request by zhuhaow.
  GitHub #54.
• Upgraded memmap2 dependency.