Eisenstein integers

[Bodigrim]

My feeling is that currently #73 is too broad and too vague. We should rather explore more special cases to hone API before taking a stab at the general problem.

We have already implemented the simplest quadratic ring, which is the ring of Gaussian integers. The next step is to handle the ring of Eisenstein integers.

The API may follow the outline of Math.NumberTheory.GaussianIntegers. Required mathematical background can be found here:

• https://en.wikipedia.org/wiki/Eisenstein_integer
• https://en.wikipedia.org/wiki/Eisenstein_prime

[rockbmb]

Sounds like an interesting issue, I'll put this in my TODO list.