Add mask<->vector conversion methods

[valadaptive]

Resolves #398.

This PR adds methods for converting masks to and from vectors.

The SimdMask trait now has an Ints associated type, which represents the mask's corresponding integer vector type.

For AVX-512, these are implemented using movm_epi[N] and movepi[N]_mask. For all other instruction sets, the mask types are the same as the integer vector types, so all we have to do is re-wrap them in the proper type.

I'm not attached to any of the names here (Ints, from_vector, to_vector) and am willing to change them if anyone has a better idea.

I'm also not sure if we should provide corresponding from_mask or to_mask functions on the corresponding integer vector types; I think that would require adding a new SimdSignedInt type or implementing this functionality for both signed and unsigned int vector types.

I also implemented these as associated methods on SimdMask but haven't added any sort of conversion trait. In general, I prefer APIs where the types are concrete if possible (no wide-ranging conversion trait) because it helps with type inference and reduces the number of "turbofish" I have to type out.

[Shnatsel]

The direction looks good to me, I'll take a closer look later. Thanks!

[Shnatsel]

std::simd calls these from_simd/to_simd: rust-lang/portable-simd#478

We generally try to align with std::simd on naming, because that's what other libs are also doing and it makes porting across the whole ecosystem really easy. But in our case to_simd sounds like it's going to give you the Simd token, so I'm not sure what the best option is.

[Shnatsel]

I've pushed a change to specify more generic bounds, cargo-semver-checks is happy with it. PTAL.

We need to make a call on whether to align with std::simd and call this to_simd or keep the to_vector, I'm OK with either.