downcast geometry array

[kylebarron]

todo:

• Change downcast to be implemented in terms of resolve_types and cast.
• Handle slicing in GeometryArray::has_points. I.e. bring back the slice offset and length. Then if the array has been sliced and the point array exists, check if the point values are within the bounds of the current slice. The fastest way to do this should be to check the length of the point array, then as you're iterating through the type_ids array, once you've seen that many ids pointing to the point array, you know the array is fully there. Or, in the cast of downcasting you really only care about whether any geometry exists. So you can short-circuit as soon as you've seen the first point.
• Ensure we apply slicing when exporting the GeometryArray. E.g. in the TryFrom impl from GeometryArray to PointArray, we need to call the slice when we export.
• recursively expand the types from the geometry collection array. But only if the geometry collection array has only a single geometry per row and it could be split up.

Closes #416