Implement `view[index]` for memory-backed `View`s
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In general, implementations of View compute their elements using an arbitrary function, but in practice many Views are just ways of addressing memory. In such cases, it would be useful to implement std::ops::Index and std::ops::IndexMut to give read/write access to the memory via Rust borrows.
Furthermore, given a memory-backed View indexed by I whose elements are Views indexed by J, v.nested() could return a View indexed by (I, J). I resisted this for Views that are not memory-backed, for fear of exploding costs.
This is now mostly done. As a witness, I've written an empty trait MemoryView which depends on View, Index and IndexMut. I've written a macro to avoid repetitive boiler-plate implementations.
I've implemented MemoryView for every View in the crate type except Diagonal and Concat which require special treatment.
View::nested() is still TODO. It would also be nice to have View::view_ref() and View::view_mut() to obtain views whose values are borrows. The former would replace Array::view().
View::view_mut() is not possible, because mutable references don't implement Clone.
There is a requirement for types such as &mut V and Arc<V> (which dereference to V) to implement MemoryView when V does. Without this feature, you cannot wrap a MemoryView without consuming it.
Unfortunately, &mut V and Arc<V> do not implement Index and IndexMut, and there's nothing I can do about that because they are defined in the standard library.
So I've had to change the definition of MemoryView. It cannot depend on Index and IndexMut. Instead, it has methods at_ref() and at_mut().
I've written a macro impl_ops_for_memoryview, analogous to impl_ops_for_view, which implements Index and IndexMut in terms of MemoryView. This can be used for new types, at least. Rust's auto-dereferencing then allows the ergonomic view[index] syntax to be used for &mut V and Arc<V>.
Work in progress!
View::nested() and View::view_ref() are done.