Create FieldElement class for Metal code

[andrewmilson]

Might be good to use this as a reference https://github.com/elibensasson/libSTARK/blob/master/algebra/algebralib/headers/algebraLib/FieldElement.hpp

Don't need all these methods. For instance, the GPU functionality doesn't require doing inverses.
Having a common "power" method will be good.