Refactor newATTRSUB()'s logic for grafting a sub definition to an existing stub

Refactor newATTRSUB()'s logic for grafting a sub definition to an existing stub

Previously it was using cv_undef() to (partially) free the target CV (the
pre-existing stub), before donating it the padlist and outside pointers from
the source CV (the definition, just compiled), and then freeing up the remains
of the source CV.

Instead, explicitly exchange padlist and outside pointers, explicitly assign
other fields that need changing (file and stash), and assert that various
CvFLAGS are as we expect them.