Fix grievous error in ISqrt function

ISqrt would get stuck if x1 was exactly 1. No bueno!
Add solutions for problems 4 and 5

I suspect the solution for problem 5 is suboptimal but it is much
better than my first iteration that worked, but was costly.

Problem 5 fell out easily from experiments by hand with the prime
factors, however I think the solution should be general.
Add solution to problem 6

Trivial in F#.