Day 7: Complete P36 and P37

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Change _isPrime, totient, primeFactors from val to def

In particular totient, so we can attempt P38.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 7 Part 2: Complete P39 to 41

Which in effect completes the Arithmetic section =)

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>