Setting up scaffolding for vectorization.

[xvzcf]

I moved the code for PolynomialRingElement into polynomial.rs, and in order to facilitate the transition towards a vectorized codebase, I added another struct to polynomial.rs called VectorPolynomialRingElement:

VectorPolynomialRingElement contains functions to convert to and from a PolynomialRingElement. This way I can incrementally make changes to the codebase; eventually PolynomialRingElement will be removed and VectorPolynomialRingElement will be take its place. An example of how this works can be seen here.

[karthikbhargavan]

The restructure looks good. The real test will be when we add a new implementation of the ring element.