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
We could either make the constructor `explicit`, which requires explicitly specifying `Point` when constructing an object of this type. If we don't, we could also write code like `map.setLatLngZoom({ settings.latitude, settings.longitude }, settings.zoom);` which has the first argument implicitly cast to a LatLng object.