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/lib.rs

@@ -255,6 +255,8 @@ mod api;
 mod codegen;
 mod sealed;
 
+pub use crate::sealed::{Simd as SimdVector, Shuffle, SimdArray, Mask};
+
 /// Packed SIMD vector type.
 ///
 /// # Examples