fiat-crypto now generates proper types, so take advantage of that.
Add mixed subtraction and double base multiplication.
We will eventually leverage mixed addition/subtraction for fixed
base multiplication. The reason we don't right now is that
precomputing the tables at comptime would take forever.
We don't use combs for the same reason. Stage2 + less function
calls in the fiat-crypto generated code will eventually address
that.
Also make the edwards25519 code consistent with these changes.
No functional changes.
Instead of multiple references to an anonymous structure to represent affine coordinates, add an actual `AffineCoordinates` structure.
Also properly handle the neutral element during coordinate conversion and fix mixed addition.
And comptime the small precomputation table for basepoint multiplication.
Uses verified code generated by fiat-crypto for field arithmetic, and complete formulas to avoid side channels.
There's still plenty of room for optimizations, especially with a fixed base. But this gives us a framework to easily add other similar curves.
