• Use a table for SV body sizes. These incorporate the space saving post 5.8.x
  
• Correctly handle SvOOK scalars. 5.12 and later don't use SvIVX().
  
• Magic vtables aren't freed when magic is freed, so don't count them.
  (They are static structures. Anything that assumes otherwise is buggy.)
  
• Reinstate 5.6.x "support". (i.e. it compiles and passes tests.)
  
• Reinstate 5.005_xx "support". (i.e. it compiles and passes tests.)