tor-geoip: Add new crate with GeoIP database functionality

• This adds a new crate, tor-geoip, which can parse and perform lookups in the GeoIP database C-tor already uses (generated by a maintenance utility in the C-tor codebase).
• We embed a copy of C-tor's databases with the crate and use include_str! to ship them with the binary, bloating its size somewhat.
  • This does, however, solve the problem of figuring out how to distribute these.
  • The plan is to gate this functionality behind a feature flag anyway, so the cost should be nil unless explicitly opted into.

Part of onionmasq#47 (closed).