t informs the bitset of a planned change in size, so that it can
manage the storage allocation accordingly. After reserve(), capacity() is greater or equal to the
argument of reserve() if reallocation happens; and equal to the previous value of capacity() otherwise.
Reallocation happens at this point if and only if the current capacity is less than the argument of
reserve(). <br />
<i>Note:</i> It does not change the size() of the bitset.<br />
<b>Postcondtitions:</b> <tt>this->capacity() >= num_bits</tt>.<