Support for ed25519 scalar field, ed25519 elliptic curve, and small refactorization

[fabrice102]

I added support for ed25519 scalar field, support for polynomials with coefficients being points of the large prime-order subgroup of ED25519, and did some refactorization (adding one layer of abstraction to support polynomials with coefficients in a prime-order group instead of a field, and adding helper functions/macros to make it easier to add new curves).

Due to the refactorization, I'm not sure whether it makes sense to do a PR or not.

Code is here: https://github.com/fabrice102/ecfft-group