Add MAYBE_DEREF_GV macro

Add MAYBE_DEREF_GV macro

There are so many parts of the core (mostly pp functions or functions
they call) that need a glob or a globref, including many that call
get-magic at the wrong time (or not at all in some cases, that it
makes sense to add a macro to do it.

It can be used like this:

   if (gv = MAYBE_DEREF_GV(sv)) /* calls get-magic */
   {

   }
   else { /* avoid magic here */

   }