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.