Projections rebase

[bergey]

This is a proposal for non-affine transformations, which I've provisionally called Projections. Before this is ready to merge, we should choose a better name for applying a Projection. The obvious choice project conflicts with Data.VectorSpace.project.

I'll add some examples on the PR for diagrams-lib.

[byorgey]

What's the rationale for this going in diagrams-core (as opposed to -lib)?

[bergey]

I put it here for consistency with Transformable. Now that you ask, I see that Transformable is in -core in part because there are several instances in -core, which isn't true of Deformable. Shall I move it over to -lib?

[byorgey]

Well, not only are there instances for Transformable in -core, the definition of QDiagram itself involves Transformations. So the transformation stuff really is an integral part of the core library. The boundary between -core and -lib is always a bit blurry, but my general approach has usually been to prefer putting things in -lib when possible, so I think the deform stuff would probably fit better in -lib.

However, we still might want to put, e.g. a note or two in the Haddocks for transformations pointing to the deformation stuff in -lib, so if people are looking at transformations and wondering why only affine are supported they will find a pointer to -lib.

[bergey]

I've moved all the code to -lib. We can add the Haddocks later, without all the mess in this branch.