Add Vec128 sort

[TheNumbat]

1. Deletes the specifc vec128 type info from the lambda vec128 layout.
   • This information was unused, and seemed non-trivial to preserve via the frontend layouts. It also wasn't really useful in the first place, since all simd operations define the relevant vector layouts.
2. Adds a new Vec128 sort and unboxed versions of our 128-bit simd vector types.
3. Duplicates the simd vector tests for unboxed vectors.
4. Adds box/unbox primitives for vec128 layout

Reviewable by commit.

Three features remain unsupported:

• Fields of unboxed vector type (Unboxed vec128 mixed block fields #3077)
• Arrays of unboxed vectors (Unboxed vec128 arrays #2969)
• String/bytes/bigstring load/store for unboxed vectors (Unboxed vec128 string/bytes/bigstring/array load/store #2966)

Currently the compiler will raise in these cases, but they will be addressed in later PRs.

[ccasin]

Really sorry that, due to ICFP prep, ICFP, and the sprint, it took me so long to get to this. Various comments below. I'd also like to talk through the story about how this will work in the tree (want to avoid jsoo issues like we had with float32), but perhaps we can talk through that when we meet tomorrow.

I've read everything under ocaml. Please also find appropriate reviewers for the middleend and backend bits. (It may make sense for them to review the tests as well, but probably you'll want to address my requests for more tests first)

[TheNumbat]

@mshinwell has looked over the changes under middle_end/flambda2

[mshinwell]

Not really, I just had a question!

[ccasin]

Approved for the front-end bits. Obviously don't merge until we're also ready to merge the PRs fixing the fatal errors.