Support reading and writing Point, LineString etc. directly

[cgrenz]

This splits WKBAbleExt into two new traits WKBSerializable and WKBUnserializable, implements WKBAbleExt for every type implementing both traits, and implements the new traits for Geometry, Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon and GeometryCollection.

With this all supported geometries can be read and written, even when not wrapped in a Geometry enum.

Additionally it adds ?Sized on all Write bounds. With this unsized writers are fully supported, without resorting to &mut &mut R as previously used in write_geom_to_wkb (mut result: &mut W combined with &mut result).

[amandasaurus]

oh that looks useful. I have learned a lot about how to writte better Rust since I created this library. I made many beginner mistakes.

Shouldn't we call it Desearalize rather than Unserealize, to be like serde?

[cgrenz]

Yeah, you're right, Deserialize is definitely better. I renamed the trait.

[cgrenz]

@amandasaurus do you think you can merge this now?