This begins to attempt to restructure the source tree so that the common parts
are common and the different parts are different.
The backend is now split into two parts:
- serial (containing the implementation using serial formulas and mixed-model arithmetic).
- vector (containing the implementation using parallel formulas and single-model arithmetic).
The serial scalar_mul tree is now under backend::serial::scalar_mul.
The avx2 scalar_mul tree is now under backend::avx2::scalar_mul.
`FieldElement32` -> `FieldElement2625`
`FieldElement64` -> `FieldElement51`
`Scalar32` -> `Scalar29`
`Scalar64` -> `Scalar52`
This naming is more accurate and would let us add an ADX backend later.
