define CvISXSUB to compile on older Perls, too
fix RT#94013 in conjunction with RT#93995

* RT#94013 fix is currently done by importing all into LMU itself as
  recommended by tobyink. Documentation about the implementations has
  been added to clarify what the implementation tags mean
* RT#93995 kindly asks for access to the export tags which are provided
  by two (undocumented) methods. Have to figure out how to support that
  in future with support soon.
prepare Makefile.PL+inc/Tumble.pm for older perls