Boolean ops should be available between differing geometry types

[urschrei]

In keeping with JTS and GEOS, boolean ops should be available between all the "official" geometries. Currently, they're only available between two geometries of the same types, and this, coupled with the union op's output of MultiPolygon, is causing difficulties in extending the API, as fold and reduce operations are greatly complicated by this asymmetry.

If / when #1089 has merged, we should revisit the public API with a view to a) extending it and b) seeing whether it's possible to change the default output of union.