Notes

Implementing separate dimension lists for each axis of each cursor (as
clones of the master dimension list) would eliminate the need to check
whether the currently visible dimension still exists during view_rotate()
and would also make it possible to hide even essential dimensions.

slice_upgrade() would have to be enhanced to check that each axis is a
loop and create the clones of the master dimension list if not.  It should
also check if the master dimension list is a loop and fix it if not.
Or maybe the routines that rely on this (eg deletion) should fix it?

It must not be legal to make or break any links +d.1 from anywhere in the
rank $CURSOR_HOME+d.2 because this is how the dimension lists are found.