Add coords() and map_coords() utils.

Coords() yields all coordinate tuples from a geometry or
feature object.

Map_coords() maps a function over all coordinate tuples and returns
a geometry of the same type. Useful for translating a geometry in
space or flipping coordinate order.

I've rewritten functions like these a number of times and hope you'll
agree that python-geojson would be a good home for them. The tests
are for pytest, which I'm loving these days (I've converted Fiona and
Rasterio to pytest, am converting Shapely as I go).