Implementing groupBy for vector.

I have stuck to O(1) operations as much as possible but the method is still O(n) because it needs to inspect every element. I think that this would be useful for other devs than myself.

Before I do any more work on this PR, what would I have to do in order to get this pull request accepted by the vector library? Is it even a feature that you want or should I just keep it lying around instead of trying to merge it back into the library?

This version of groupBy is [based on the one in the Prelude][1] but with modifications to take advantage of the fast operations in the vector library as opposed to Data.List.

 [1]: http://hackage.haskell.org/package/base-4.7.0.0/docs/src/Data-List.html#groupBy