Solve problem 9
Solve problem 10

This changes the isPrime function, which:

- Has significantly improved performance (with a bit of help from http://holmezideas.com/programming/optimization-techniques/fastest-algorithm-to-check-if-a-given-number-is-prime/)
- Considers 2 a prime number now (before it incorrectly did not)
- Reveals an error in problem 7 (error has been fixed)
- Is now in a separate functions.h/.c file
Solve problem 11