Add a serde feature-flag that adds (De-)Serialize impls for public types

[vkahl]

I've got a use case where I'd like to store a Projection (created using Projection::from_control_points) in a database. Of course I can just store the control points and recalculate the projection every time I need it, but it would be much nicer to just serialize the Projection itself. A middle ground solution would be to store the transform and recreate the Projection using from_transform (which is way less expensive than from_control_points), but there's no accessor for the transform field, so that doesn't work either. The runtime cost of from_control_points is probably cheap compared to the image processing that happens afterwards, but adding a serde-feature would also reduces complexity/friction in downstream projects like mine and is kind of a convention in the rust ecosystem, so I think it's worth considering.