Implement the sealed trait pattern from #258

This implements the solution proposed in #258 to allow generic usage of
`Simd<T>` without comprimising the safety of `Simd<T>`.

Author: RustyYato

src/codegen.rs

@@ -16,13 +16,16 @@ macro_rules! impl_simd_array {
         pub struct $tuple_id($(crate $elem_tys),*);
         //^^^^^^^ leaked through SimdArray
 
+        impl crate::sealed::Seal for [$elem_ty; $elem_count] {}
+
         impl crate::sealed::SimdArray for [$elem_ty; $elem_count] {
             type Tuple = $tuple_id;
             type T = $elem_ty;
             const N: usize = $elem_count;
             type NT = [u32; $elem_count];
         }
 
+        impl crate::sealed::Seal for $tuple_id {}
         impl crate::sealed::Simd for $tuple_id {
             type Element = $elem_ty;
             const LANES: usize = $elem_count;