Simplify the vectore reversion in conv

Hi!

I've noticed that the convolution function uses intermediate steps to convert to matrix and back in order to flip a vector. Importing Data.Vector.Storable and using Vector.reverse directly introduces a ~30% speed gain.
I've prepared a benchmark to show this: https://gist.github.com/erdeszt/a932dbf83daf0b4e0c6b
The pr fixes the issue.