This change required some work, because the to-be-stabilized SIMD functions
don't allow non-constant `imm8`s. Previously, the `stdsimd` functions had a
constifying macro that ensured that the immediates were known. The dalek code
used this to build helper functions which would be inlined into different
places where the immediates were known. Unfortunately, since constexprs aren't
fully supported in Rust yet, this is done by a hidden compiler attribute, and
there's no way to propagate "constness".
To deal with this, some of the functions are specialized (e.g.,
`square_and_negate_D` instead of taking a mask), and others use an enum.
Since Criterion can only benchmark public API, these changes just drop
all internal benchmarks (e.g., benchmarks for field operations). But
those are usually microbenchmarks whose meaning is kind of questionable
anyways, so I don't think this is a big loss.
The `bench` feature disappears, since Criterion works on stable Rust.
A missing import of the Borrow trait caused the build to break with the
"yolocrypto" feature enabled; this was't caught by CI because the CI machine
that Travis used didn't have AVX2, so the code was never built.
This commit adds the missing import and changes `std` to `core` so that the
AVX2 backend builds with no_std, but this isn't tested and is, actually,
"yolocrypto".
