All structs should use same struct basis

[RokLenarcic]

This fixes the problem where each row uses a new struct basis object which is not correct. It works but it wastes time and memory. The basis objects are retained with the maps which can be seen here, using clj-memory-meter:

Before this patch:

(mm/measure
  (slurp-csv "/Users/roklenarcic/Downloads/analysis20230215064100/age.csv"
             :mappify true
             :structs true))
=> "6.5 MiB"
(mm/measure
  (slurp-csv "/Users/roklenarcic/Downloads/analysis20230215064100/age.csv"
             :mappify true
             :structs false))
=> "4.8 MiB"

After this patch:

(mm/measure
  (slurp-csv "/Users/roklenarcic/.....csv"
             :mappify true
             :structs true))
=> "4.8 MiB"
(mm/measure
  (slurp-csv "/Users/roklenarcic/.....csv"
             :mappify true
             :structs false))
=> "4.8 MiB"