P1684R5: Conversions and `to_mdspan` should provide untemplated overloads

[miscco]

Currently the conversion to mdspan and the to_mdspan are templated with different template arguments

  template<class OtherElementType, class OtherExtents,
                   class OtherLayoutType, class OtherAccessorType>
  operator mdspan ();

  template<class OtherAccessorType>
    constexpr mdspan<element_type, extents_type, layout_type, OtherAccessorType>
      to_mdspan(const OtherAccessorType& a = default_accessor<element_type>());

That prevents us from using CTAD for the common use case where one converts to a matching mdspan. We should provide untemplated overloads so that this works