cpp/container/mdspan/extents

@1@ Represents a multidimensional index space of rank equal to. @2@ A convenient alias template for an all-dynamic. Let denote, each specialization of it  is equivalent to.

Each specialization of models  and is.

Data members
Typical implementations hold only one non-static data member:
 * of type, which stores the size of each dynamic extents. The name is for exposition only.