Stop exporting Diagrams.Coordinates from Diagrams.Prelude

To my knowledge there are two operator clashes between diagrams and lens: (&) and (#).  (#) is fundamental to diagrams but quite peripheral to lens, so I'm happy to leave things as they are---users of diagrams+lens can hide (#) from their lens import.

With (&), the situation is reversed --- it's used heavily in idiomatic lens code, but in the context of diagrams it isn't always needed, and when it is one can just as easily use functions like `r2` and `p2`.  The proposal, therefore, is to stop re-exporting `Diagrams.Coordinates` (and hence (&)) from `Diagrams.Prelude`.  Users who want it can still import `Diagrams.Coordinates` explicitly.