CRS trait

[kylebarron]

I wonder if CoordinateReferenceSystem would make a good trait in a future PR (could be backed by a string or by a Python object or by a constant struct like struct LngLat{}, with methods for equality and serialization)

Originally posted by @paleolimbot in #1049 (comment)

[kylebarron]

I haven't thought much about the CRS API. Open to suggestions here.