Use generic trait parameters to minimize associated types

[calebzulawski]

This improves type inference by removing most associated types. For example, the following function doesn't actually work:

fn foo<T: SimdElement>(x: Simd<T, 4>) -> Mask<T::Mask, 4>
where
    Simd<T, 4>: SimdFloat,
{
    x.is_sign_negative()
}

You actually need the constraint Simd<T, 4>: SimdFloat<Mask = Mask<T::Mask, 4>>.

With this PR, the function should look a little more like:

fn foo<T: SimdElement>(x: Simd<T, 4>) -> Mask<T::Mask, 4>
where
    Simd<T, 4>: SimdFloat<T, 4>,
{
    x.is_sign_negative()
}

There are still a few instances that need associated types, but we can explore those individually to see if there's a better API.

Also, note that we now end up with a lot of instances of Mask<<T as SimdElement>::Mask, N>. We can either remove the SimdElement::Mask associated type with #483 or we can provide an alias to avoid writing that out.