Update interface.rs
Update interface.rs

Changes to `encode`
* Use arithmetic operators
* Reserve String space as done in `/cpp/openlocationcode.cc`
bocops/open-location-code,google/open-location-code,google/open-location-code,bocops/open-location-code,google/open-location-code,google/open-location-code,zongweil/open-location-code,google/open-location-code,google/open-location-code,zongweil/open-location-code,google/open-location-code,bocops/open-location-code,zongweil/open-location-code,bocops/open-location-code,zongweil/open-location-code,zongweil/open-location-code,zongweil/open-location-code,zongweil/open-location-code,zongweil/open-location-code,bocops/open-location-code,zongweil/open-location-code,bocops/open-location-code,zongweil/open-location-code,bocops/open-location-code,bocops/open-location-code,zongweil/open-location-code,google/open-location-code,google/open-location-code,bocops/open-location-code,bocops/open-location-code,google/open-location-code,google/open-location-code,bocops/open-location-code