Add IO functions to read and write WKT and WKB

[jorisvandenbossche]

We should have functions to create geographies from WKT and WKB, and also to convert to WKT and WKB.

(similarly to shapely's from_wkt/from_wkb and to_wkt/to_wkb)

At the moment s2geography does have a basic WKTWriter (which we already use in __repr__) and WKTReader, but proper (and easier) support probably depends on https://github.com/paleolimbot/s2geography/pull/16