[views] Subclauses for contiguous and multidimensional access

This organises the material for <span> and <mdspan> into separate
subclauses, rather than having all constituent parts being siblings in
the same subclause. Even though we now have some subclauses six levels
deep, this seems like an improvement. As a drive-by, this allows us to
move a subclause on a non-member span helper function up one level,
where it is more appropriate.

Fixes NB FR 027 (C++23 CD).

Author: tkoeppe