Flatten GeometryCollection into non-complex geometries

[Ilya Shumeyko]

I ran into a need to flatten complex GeometryCollection into a non-complex MultiPolygon geometries.

I'm wondering if there is a place in geo for this feature (maybe, lib/geo/utils.ex?). Or probably it would be a better fit for https://github.com/pkinney/topo?

I'd be happy to contribute some code, just want to make sure it fits the project needs.