MGLFeature should contain MGLShape, not inherit from it

[1ec5]

MGLFeature should be a class, not a protocol, and it should contain an MGLShape in its shape property. The various MGL*Feature classes that conform to the MGLFeature protocol should go away. MGLShapeCollectionFeature should be renamed to MGLFeatureCollection. MGLShape, MGLFeature, and MGLFeatureCollection should conform to a new MGLGeoJSONRepresentable protocol.

In #5110, I needed to add classes for all the possible feature types that could be returned from the feature querying API. I made a series of MGL*Feature classes: each class conforms to the MGLFeature protocol and inherits from an MGLShape class (which corresponds to a GeoJSON geometry type). For example, MGLPolygonFeature inherits from MGLPolygon. By contrast, in the GeoJSON specification, a feature contains a geometry; it isn’t a kind of geometry.

I don’t remember if the taxonomy I implemented was a misreading of the GeoJSON specification on my part or a deliberate shortcut, but it proved to be very convenient for working with feature querying results. You can take the feature querying results and turn right around and add those results to the map as annotations (since some MGLShape types are annotation types). Unfortunately, this taxonomy works less well with the runtime styling API: for #7453, what happens when an array of MGLShapes is really a mixture of shapes and features?

The proposed refactoring mirrors the GeoJSON specification exactly and removes the rough edges around the runtime styling API.

/ref #5201 #7160 (comment)
/cc @boundsj @frederoni @jfirebaugh

[1ec5]

Since +[MGLShape shapeWithData:encoding:error:] could return any instance of a class conforming to MGLGeoJSONRepresentable, not necessarily an MGLShape, we’ll need to pull out this method and -[MGLShape geoJSONDataUsingEncoding:] into a new class, MGLGeoJSONSerialization.

[1ec5]

The diagram in #7454 (comment) isn’t quite accurate: only MGLPolyline and MGLPolygon inherit from MGLMultiPoint; the rest of the geometry classes inherit directly from MGLShape. #12419 proposes eliminating MGLMultiPoint to align MGLPolyline and MGLPolygon with the rest of the geometry classes.

/cc @captainbarbosa