Generic instances needed

[leftaroundabout]

The TensorSpace, LinearSpace and FiniteDimensional classes are pretty bloated, with rather technical methods. It would be very cumbersome having to define all those instances for every custom vectorspace. It shouldn't be necessary either – the definitions seem automatic enough to make it feasible with GHC generics, so a new compound vector space (in particular records, which are basically just direct sums) can be given an empty LinearSpace instance (or directly deriving one, with -XDeriveAnyClass).