Make order of arguments for groupby consistent with other iterators.Pull request/95d755dd

This switches the order of the function and collection arguments in `groupby` so that it is consistent with the  `imap` and `iterate` iterators and higher order functions in Base such as `map`. A warning is issued if `groupby` is called with arguments in the old order. 

Also, the `GroupBy2` benchmark was not being run, and I assume that was omitted by mistake. `GroupBy2` is now run.