Problem 43 - setup

Extended the pandigital accumulator and sequence generator to
work with an arbitary starting digit, as long as it is less than
the final digit. This allows them to work with 1 to 9 as well as
0 to 9 pandigital numbers.
Problem 43 - solution

Vastly improved the performance of the pandigital sequence generator
by making it work off of permutations instead of numerical
iteration.

This revision also makes the solution to problem 41 fast.