Deprecate LineString::coords and LineString::coords_mut?

[callpraths]

Related: #706 #708

None of the other geo-types have equivalent methods. There are alternatives in geo::algorithms that are more uniformly implemented:

• LineString::coords: Alternative is CoordsIter::coords_iter (although that returns an iterator that copies the individual coordinates rather than returning a reference; If necessary, we could add a version of CoordsIter::coords_iter that returns references.
• LineString::coords_mut: Closest alternative is MapCoordsInplace::map_coords_inplace. If we think we still need mutable iterators, that's another one we could add to CoordsIter uniformly for all types.