Problem 32 - Solution 1

The accumulator part is not tested.
This solution is not algorithmically performant enough as the
generation of pandigital numbers takes too long.
Problem 32 - A little better

But still not fast enough.
Problem 32 - Wrong solution

Uses an in-place permutation function to construct the pandigital
string. Had to include it inspite of the bug as it might be
useful later.
Problem 32 - Solution

Traded the permutation approach for a search method. The search
is narrowed down by recognizing that the two multiplicands cannot
be that large really;
Problem 32 - Solution 2

Came back to the permutation idea after realizing that a 9-digit
pandigital number does not include zero.

Also found a built-in permutation function in std.algorithm so
no need to reinvent the wheel.