Use global-scope traits instead of nested type aliases for mdspan concepts

Requiring the definition of nested type aliases in a concept is annoying, because you can't provide a "default" definition, so type authors implementing your concept must define the nested type aliases. This pattern exists mostly for historical reasons:

• Pre C++11, these nested type aliases were often an unfortunate necessity (Boost/TR1 result_of and (unary|binary)_function, etc). Post C++11, this is frequently syntax sugar as these types can be discovered via decltype, etc.
• Pre C++11, using separate traits class customization points to define these types was unwieldy because we didn't have type aliases to save us from typename (e.g. typename X<T>::type instead of X_t<T>).

Iterators, containers, and allocators all suffer from this problem.

In modern C++, there's a better solution: define a traits class for each nested type aliases (not a single traits class for all the types, like iterator_traits or numeric_traits), provide a reasonable default, and allow users to customize it. For example:

template <typename MyConcept>
struct X { using type = /* default */; };

template <typename MyConcept>
using X_t = typename X<T>::type;

The executors proposal uses this pattern (executor_future_t, executor_index_t, etc). We should do the same for mdspan concepts like Accessor and Layout.