This saves 4 point doublings by unwrapping the first loop iteration,
as well as 63 field multiplications (one per iteration) by managing
curve model choice explicitly.
This doesn't (yet) give any speedup over the non-precomputed multiscalar
multiplication, and it's not clear that it's a good idea to commit to
supporting it in the future. Removing it means that it's not committed-to as
part of the public API, but the source is still there in the tree if we want to
revisit it later.
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.
