Stub for coordinate transformers

[paleolimbot]

Probably a small C struct in the header that defines a forward and backward transformation + a domain and range + dimensions in/out. This concept is needed in S2 to support tessellation and would add affine transform to wk. It would also make projections easier to implement and would make tools that work with projections (e.g., adaptive resample, tessellation) easier to generalize. Note that S2 needs the range/domain to implement coordinate wrapping.

[paleolimbot]

Was done in #88, #89