[perl #48014] [DEPRECATED] PMC union struct

[Christoph Otto via RT]

I'm running into a snag trying to implement this.  It turns out that
many lines which use the PMC_x_val macros use them on different types of
PMCs, especially parents and children (e.g. FixedPMCArray and
ResizablePMCArray).  There are also some instances where the PMCs with a
related purpose but no inheritance (e.g. RetContinuation and
ExceptionHandler) have the unionval used in the same way on a line of code.

What's the right way to fix these cases to use the GETATTR/SETATTR macros?

I have a file that lists these, which I'll be sticking on the wiki later
tonight.