[RFC O/2] libstdc++: Implement PL007 changes to submdspan

[Tomasz Kamiński]

This patch implements the changes suggested in PL007 NB comment
for C++26. This is separated in two patches:
* PATCH 1/2 - introduces range_slice type, without modifying
  strided_slices. This allows comparison of behavior before
  and after changes.
* PATCH 2/2 - modifies the meaning of strided_slice::extents,
  and expand support of decomposable types