Add From impl for lossless casts between primitive types and vector types

[gnzlbg]

Example: From<i32> for i64x2:

let v: i64x2 = 42_i32.into();
assert_eq!(v, i64x2::splat(42));

Note: impl<T, const N> From<T> for Simd<[T; N]> is just Simd::<[T; N]>::splat(T).