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

[ds84182]

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
}

[clarfonthey]

Not to bikeshed too hard, but I would prefer e.g. u32x<4> over u32xN<4>.

[programmerjake]

the only issue with that is if we added scalar types named like f64x meaning f64-extended (f80 or similar) which could then be confused with the type alias for Simd<f64, N>

[calebzulawski]

Can type take stabilization attributes? Should be uncontroversial to add something like this behind a separate feature and deal with it later