hail/allocate: implement multi-group support

This is a bit hackish. We add a new function that takes the input data,
splits it into groups, runs the original tryAlloc for each group, and
then chooses the best solution, but adds the log messages from all the
groups, as to give better debugging information. In hail, we just point
to this new function.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>