improve pretty printing of shapes

We should now consistently get:
  (Z :. x :. y)
rather than:
  ((Z :. x) :. y)
a poor choice of names
simplify shapeSize of constant
Merge branch 'master' into sequences

Conflicts:
	Data/Array/Accelerate/Pretty/Print.hs
	Data/Array/Accelerate/Trafo/Simplify.hs

The pretty printing for the union operation needed to be updated to match the better printing of shapes.