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.
Change Scalar::non_adjacent_form() to take a width parameter.
This rewrite also makes it faster, although it's probably a ways off
from optimal. I don't know how much it matters.
TODO: write up description of why this computes the same thing.
Thanks to @oleganza for pointing out an error reading bits across words
in an earlier version of this code.
