Problem 6 in C.  Inefficient calculation of sum of squares
Problem 6 in C solved WITH THE POWER OF MATHS\!
Generate primes using the Sieve of Eratosthenes for problem 7.

Decent implementation in C but has some downsides:
 - It's inefficient with space
   - we don't need to store even numbers
   - we don't need a whole byte just to store a flag
 - It's probably ineffient in time
   - The iteration over the even numbers is pointless if we don't need to store them
 - It needs to be factored out into a library