Solution to problem 3

Solution to problem 3 using a prime sieve. I am unhappy with the result
though, as it seems a fraction too slow for my liking.
Slow solution to problem 4

I think this is a slow solution to 4, since it requires sorting lists a few
times in order to get the result we need, since otherwise it'll get the
palindrome from the first product, which may not necessarily be the largest.