Set the number of buckets in the table. This method might be useful for
two purposes:
When the number of elements in known in advance, setting the number
of buckets avoids rehashing.
The number of buckets is not minimised if members are deleted or the
table is cleared. The hash_table->buckets
method may be used to make the table smaller.
The next legal value (large enough and a power of 2) is used.