Sequential integers would be densely packed in the table, leaving the
high-index buckets unused and causing abnormally long probes for many
operations. This was especially noticeable with failed lookups and
when "aging" the table by repeatedly inserting and removing integers
from a narrow range using g_direct_hash() as the hashing function.

The solution is to multiply the hash by a small prime before applying
the modulo. The compiler optimizes this to a few left shifts and adds, so
the constant overhead is small, and the entries will be spread out,
yielding a lower average probe count.