Add file to construct fields in Sage

[divergentdave]

This adds a Python file that handles constructing Sage finite field objects, for use throughout the rest of the reference implementation. This is similar to the code from #32 (comment), but with ^ and .<x> desugared, and using Python ints instead of sage.rings.integer.Integer. This module doesn't provide generators as exported module variables, to avoid confusion between variable names. Generators are instead only created inside function scopes. Other code that needs to access a generator element can call the field's gens() method later.

[divergentdave]

I changed my mind on the name, I want to call this module fields to allow for arguments named field without shadowing variable names.