Move generic BigInteger numerical algos into separate module

I wanted to start to get my head around module syntax and how to
structure project solutions in Xamarin studio.

Miscellaneous code clerking is interspersed.
Add a signature for BigInt and a readme

Add signature file for BigInt module -- mostly to learn the basic
syntax and how to organize it in the ide (must come before module
itself in file listing). This seems to quell some false positive
warnings about BigInt being an undeclared module.

Some trivial code clerking.
Solution for problem 7

This solutions is slow and uses a lot of space. Perhaps an
infinite prime generator using the Miller-Rabin primality test would
be faster.