Fix for GitHub issue 9 part 1 : Efficiency of vvp_vector2_t::pow() function.

The vvp_vector2_t::pow() function is recursive, and performs a multiplication
operation on each step. The multiplication operator was expanding the result
vector to accomodate the maximum possible result value for the given operand
vectors, thus causing the execution time of the power operation to be
exponentially proportional to the exponent value. Both in this case and
in general, it is unnecessary for the multiplication result vector to be
expanded, as the compiler has already determined the required vector width
during elaboration, and sizes the operand vectors to match.
Fix for GitHub issue 9 part 2 : Efficiency of the verinum pow() function.

This changes the verinum pow() function to use the more efficient algorithm
used in the vvp runtime. It will still be slow if the left operand is unsized
and the right operand is large, as it will expand the result vector to avoid
overflow.