Calculate optimal buckets

[Chris Warburton]

Comparing against random bucketing is all well and good, but it would be
nice to see how good bucketing *could* be.

This is quite tricky to pin down, since we could just throw everything
into one bucket and get 100%. Hence it makes more sense to calculate the
optimal for a given *maximum bucket size*.

This probably requires a lot of search, so it might be best calculated
in a logic language like Mercury.