ToXYZ geometry conversion methods

[peterstace]

@albertteoh suggested having geometry conversion methods on the Geometry type that return a bool indicating if the conversion was successful in #399 (comment)

For example, we would add a new method to the Geometry type with signature ToPolygon() (Polygon, bool).

We should also consider whether we want to rename the AsFoo (e.g. AsPolygon) methods to make it clearer that they panic if invoked incorrectly (i.e. without first checking IsFoo). An example of a better name might be MustAsFoo.