Problems 8, 9

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>
utils: arraySum and arrayProd

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>
Problem 10, rewrite primesUnder with eratosthenes sieve

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>
Problem 11

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>
Problem 12, utils.divisors

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>