Conversion to iD-SDK: remove utility 'extent' method and replace.

[Bonkles]

As part of the cut-over to the iD SDK, we need to start excising duplicated code from the RapiD codebase and increasing dependencies on the SDK.

This is one step in that process- the untyped geoExtent tuple pair (signifying a min lat/lng and max lat/lng box for an extent of the map surface) will be replaced with a full-fledged object called Extent.