Merge pull request #142 from rust-lang/feature/traits

Combine LanesAtMost32 and SimdArray into a single trait "Vector"

Attempts to fix some unresolved questions in #139 regarding `SimdArray` having a generic parameter.

In particular, this made it not appropriate for replacing `LanesAtMost32`.  Additionally, it made it impossible to use in a context where you otherwise don't know the lane count, e.g. `impl Vector`.

An unfortunate side effect of this change is that scatter/gather no longer work in the trait (nor does anything else that references the lane count in a type.  This requires the super-unstable `const_evaluatable_checked` feature).

I also threw in the change from `as_slice` to `as_array` as discussed in zulip, and fixes #51.

crates/core_simd/src/comparisons.rs

@@ -1,13 +1,11 @@
-use crate::LanesAtMost32;
+use crate::{LaneCount, SupportedLaneCount};
 
 macro_rules! implement_mask_ops {
     { $($vector:ident => $mask:ident ($inner_ty:ident),)* } => {
         $(
             impl<const LANES: usize> crate::$vector<LANES>
             where
-                crate::$vector<LANES>: LanesAtMost32,
-                crate::$inner_ty<LANES>: LanesAtMost32,
-                crate::$mask<LANES>: crate::Mask,
+                LaneCount<LANES>: SupportedLaneCount,
             {
                 /// Test if each lane is equal to the corresponding lane in `other`.
                 #[inline]