void reserve( size_type count );
Sets the number of buckets to the number needed to accomodate at least
count elements without exceeding maximum load factor and rehashes the container, i.e. puts the elements into appropriate buckets considering that total number of buckets has changed. Effectively calls rehash(std::ceil(count / max_load_factor())).
|count||-||new capacity of the container|
 Return value
average case linear in the size of the container, worst case quadratic.
 See also
| reserves at least the specified number of buckets.|
This regenerates the hash table.
(public member function)