isPrime and factorize are slower than needed

[hypirion]

isPrime and factorize generates every prime number below the square root of the input argument. We can get away by creating elements when needed, not at the beginning.

However, this will lead to a recursive dependency on isPrime and addPrime. One has to be sure this doesn't lead to any infinite loops, though it seems likely that it should converge at some point. (Perhaps it already does, I'm not sure.)