Start to align native API with JS

* Introduce Point and LatLng classes
* Switch from lon, lat to lat, lng

Open issues:

* Requires equivalent cocoa changes
* Should we replace Point add/sub/mult/div with operator overloading?

Next up:

* Make implementation match JS (use Point more, etc.)
* Match JS function names and signatures for easings
I'm going to make them explicit. Needing to name `LatLng` when constructing provides a reminder of the correct order.