Implement kani::Arbitrary for Wrapping, Saturating, and Simd types

[carolynzech]

Implement kani::Arbitrary for the Wrapping, Saturating, and Simd types.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tautschnig]

Changes as proposed below (with an additional

#[cfg(kani)]
use crate::kani;

at the top of the files) appear to work for me.

[tautschnig]

With the changes as proposed above plus an additional

--- a/library/portable-simd/crates/core_simd/src/vector.rs
+++ b/library/portable-simd/crates/core_simd/src/vector.rs
@@ -1,3 +1,5 @@
+#[cfg(kani)]
+use crate::kani;
 use crate::simd::{
     LaneCount, Mask, MaskElement, SupportedLaneCount, Swizzle,
     cmp::SimdPartialOrd,
@@ -99,6 +101,7 @@ use crate::simd::{
 // directly constructing an instance of the type (i.e. `let vector = Simd(array)`) should be
 // avoided, as it will likely become illegal on `#[repr(simd)]` structs in the future. It also
 // causes rustc to emit illegal LLVM IR in some cases.
+#[cfg_attr(kani, derive(kani::Arbitrary))]
 #[repr(simd, packed)]
 pub struct Simd<T, const N: usize>([T; N])
 where

we get 1039 further harnesses.