ArrayView as Custom DST

[jturner314]

It would be really nice if we could replace ArrayView/ArrayViewMut with custom dynamically-sized types. RFC 2594 is a current proposal for custom DSTs. The current version would work for const-dimensionality views but not for dynamic-dimensionality views (IxDyn).

I've asked about the dynamic dimensionality case and proposed a couple of ideas. @bluss Maybe you have some more ideas?

[bluss]

Just to note, it seems like the dynamic dimensional kind of view can never be a DST (due to the dynamic size and heap allocation of the metadata i.e the sizes and strides). That doesn't have to stop us from pursuing the rest.

[akern40]

While this is very clearly the dream, custom DSTs were postponed in 2021 and there's not even a mention of them on the 2024 roadmap. With #879 coming in, we at least alleviate some of the pressure for DSTs and there's a clear path forward for unifying ArrayView(Mut) with ArrayRef if custom DSTs pop their heads back up.