Fix shape concatenation in Feldspar.Vector.MultiDim plus some other small
things.
Fix up slices of shapes in Feldspar.Vector.MultiDim.
Slices have their own data type now to replace the class that was in Repa.
The constructors of the Slice data type has to be different than those in
the Shape class and this means that specifying Slices looks different from
what it looks like in Repa. I've currently gone for a syntax which looks
rather similar but it might come across as confusing when trying to write
slices.