with_names numbered macros 0..N, and stash::set_many refuses to write a
hidden name beside a visible one of the same name, so a visible
_gc_macro_0_0 (from a file, or defined after the file opened, when the
stored names are no longer in the workbook) made every xlsx save of a
workbook with macros fail. The index is GridCraft's choice: macros now
take the free indices from 0 up, skipping every key a visible
workbook-scope name of the storage shape uses (compared as the stash
reads names), in their order. No arithmetic depends on a number from
the file.