Minor changes to group. Replaced `segmented` with Iter::partition(), added comments.
Minor changes to group. Replaced `segmented` with Iter::partition(), added comments.

Signed-off-by: Andrea Lattuada <47caf6dfbb3d1b54af5bae3cc6243963d5b33a18@inf.ethz.ch>

frankmcsherry/differential-dataflow,placrosse/differential-dataflow