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.
The serial (`u32`/`u64`) implementations use a multiple curve models, passing
between extended and projective coordinates when performing addition and
doubling (respectively). But the AVX2 backend doesn't, so in order to write a
single scalar mult implementation, we have to either abstract over the curve
models or have two implementations.
A generic solution is possible but extremely unreadable: the scalar mul
implementation would be parameterized over the point types used by the serial
implementations, with many where clauses describing how the types relate. The
AVX2 types could then be substituted in the appropriate places.
Instead we just duplicate the code into the `avx2` backend.
