Add `{u32,f32,...}xN` shorthand types for `Simd<{u32,f32,...}, N>`

Shorthand types that are generic over lane count:

type u32xN<const N: usize> = Simd<u32, N>;
type f32xN<const N: usize> = Simd<f32, N>;
type u64xN<const N: usize> = Simd<u64, N>;
type f64xN<const N: usize> = Simd<f64, N>;
// etc.

I've found it easier to write these compared to Simd<u32, N> (which I find quite noisy).

And possibly the inverse, although not as useful(?):

type Tx1<T> = Simd<T, 1>;
type Tx2<T> = Simd<T, 2>;
type Tx4<T> = Simd<T, 4>;
type Tx8<T> = Simd<T, 8>;
// etc.

This would be used in code that is generic over lane count, for example:

fn multiply_add<const N: usize>(x: f32xN<N>, y: f32xN<N>, z: f32xN<N>) -> f32xN<N>
  where LaneCount<N>: SupportedLaneCount
{
  x * y + z
}

Start with the proposed u32xN and f32xN aliases and the possible TxN inverse aliases in the issue. Determine which shorthand forms belong in the portable SIMD API and how they should behave for generic lane counts. Done means the accepted shorthand types are exposed with suitable coverage for the shown generic usage.