Reduce load factor (from 66% to 60%) to improve effectiveness of linear probing.

Reduce load factor (from 66% to 60%) to improve effectiveness of linear probing.
Decreased density gives better collision statistics (average of 2.5 probes in a
full table versus 3.0 previously) and fewer occurences of starting a second
possibly overlapping sequence of 10 linear probes.  Makes resizes a little more
frequent but each with less work (fewer insertions and fewer collisions).