MonadPrimes

Test using monads to factor number

```
primes = [2, 3, 5, 7, 11, 13, 17]
factorOnce :: Int -> [Int]
factorOnce n = case factor of Just d -> [d, div n d]
otherwise -> []
where
factor = L.find (\m -> mod n m == 0) primes
factor :: [Int] -> [Int]
factor ns | length ns' == length ns = ns
| otherwise = factor ns'
where
ns' = L.filter (>1) (ns >>= factorOnce)
```