stop exporting Coordinates from Diagrams.Prelude

This is for compatibility with lens, as (&) is a rather important lens
operator and clashes with Coordinates.  Users who want the Coordinates
stuff can import Diagrams.Coordinates explicitly.  Still need to
update documentation to note this, and fix user manual examples,
gallery, etc.
note in documentation that Diagrams.Coordinates must be explicitly imported
Merge pull request #82 from diagrams/coordinates-noexp

Stop exporting Diagrams.Coordinates from Diagrams.Prelude