Solution 21 /w separate factors module

I suspect I'll keep having to use factors, so I broke the factors stuff
out into it's own module.
Solution 22

I just did this one the easy way now that I have several solutions
(e.g. 16) that can separate a large number into several smaller numbers
with carry operations. I suppose I could create a module that does that
but with Erlang it's unnecessary.
Solution 23
Solution 24