Introduce CommonCore.Numerics

Some good stuff in here, but it is going to need a good amount of clean up. Definitely worth looking into.

• Combinatronics.Collections.*
• BigRational
• DiceMath
• MathHelper - d5c20e94
• MathHelper.Factorial - d5c20e94

The namespace should be changed from Math to Numerics, so that it doesn't collide with System.Math class.

MathHelper, as well as all the math related sequence code, such as what was in MathHelper.Factorial, has been rewritten from the ground up and incorporated across appropriate classes in the Numerics namespace.

DiceMath should be renamed DicePool to better reflect its purpose.