Migrate response parsing to Turf

This library should add the Swift port of Turf as a dependency. GeocodeResult and much of the Placemark struct’s Codable conformance should be replaced by FeatureCollection.

Rationale

This library has a significant amount of response parsing code. Even parsing something as simple as a set of routable points is a hassle:

https://github.com/mapbox/MapboxGeocoder.swift/blob/9dcd38e8bfb2933912f35cd9ecb6ae6ed97565d6/Sources/MapboxGeocoder/MBPlacemark.swift#L453-L460

The existing code is functional, but it requires manual updates any time the Geocoding API changes. Supporting a companion API such as the Japan Search API requires extra care just to get something functional.

As it happens, both APIs’ response formats are valid GeoJSON by design. The Swift port of Turf contains full-fledged, well-tested GeoJSON parsing functionality and model types. MapboxDirections successfully migrated to Turf for its geometry-related needs in mapbox/mapbox-directions-swift#382.

This library would continue to be a lightweight, maintainable wrapper atop the search APIs for use cases that don’t require the full heft of the search SDK or that require specialty endpoints like batch geocoding. Conversions to other standard iOS libraries like Contacts would remain, but the smaller codebase would make it easier to support additional Swift platforms like Linux and Windows with negligible overhead.

Implementation notes

Much of the metadata for each result is stored in GeoJSON foreign members, which are now supported by Turf: mapbox/turf-swift#175.

Turf doesn’t bridge backwards to Objective-C. In order for this library to retain Objective-C support, it would need to wrap any Turf types in Objective-C-compatible classes. That might not be much of a problem in the short term, since the response parsing is mostly an implementation detail. However, to achieve the long-term maintainability gains described above, we may want to limit Objective-C support to the current functionality or even drop it in future versions.

/ref mapbox/MapboxStatic.swift#112
/cc @OttyLab @tsuz

Start with Sources/MapboxGeocoder/MBPlacemark.swift, especially the response parsing around the linked lines, and inspect the MapboxStatic.swift reference at line 112. Compare the existing GeocodeResult and Placemark Codable handling with Turf’s FeatureCollection support, then verify that the intended search responses and the stated Objective-C compatibility scope are covered.