make RC-stack-aware: pp_readline(), pp_glob() etc

Remove the temporary wrappers from:

    pp_readline()
    pp_rcatline()
    pp_glob()
    pp_rv2gv()

and dependent functions.

This started out as an attempt to unwrap the simple pp_rv2gv() function.
But pp_readline() calls pp_rv2gv() too, so that had to be unwrapped too.
Then the bulk of pp_readline()'s implementation is done by
do_readline(), so that had to be updated too. But pp_rcatline() and
pp_glob() also call out to do_readline(), so they had to be fixed too.
And pp_glob() outsources most of its work to the XS module File::Glob,
so calling that had to be wrapped to handle a non-refcounted stack in
the XS code. Then it turns out that code in IO.xs calls pp_readline()
directly and needed tweaking.

So now its a large commit that updates nearly everything in one big go.

While fixing up everything, I took the opportunity to add many code
comments and asserts to better document what these various functions do,
and what args they expect on the stack under what flag conditions.

The op_flag processing is now more strict, so potentially other code
which directly fakes up an op with sloppy flag settings and calls one of
these pp functions directly might now trigger an assert failure. (See
the fix-up to IO.xs a couple of commits ago for an example.)

Author: iabyn

gv.c

@@ -4195,6 +4195,13 @@ Perl_amagic_call(pTHX_ SV *left, SV *right, int method, int flags)
                 res = newSV_type_mortal(SVt_PVAV);
                 av_extend((AV *)res, nret);
                 while (nret--)
+                    /* Naughtily, we don't increment the ref counts
+                     * of the items we push onto the temporary array.
+                     * So we rely on the caller knowing not to decrement them,
+                     * and to empty the array before there's any chance of
+                     * it being freed. (Probably should either turn off
+                     * AvREAL or actually increment.)
+                     */
                     av_store((AV *)res, nret, POPs);
                 break;
             }

pp.c

@@ -186,9 +186,10 @@ S_rv2gv(pTHX_ SV *sv, const bool vivify_sv, const bool strict,
     return sv;
 }
 
-PP_wrapped(pp_rv2gv, 1, 0)
+
+PP(pp_rv2gv)
 {
-    dSP; dTOPss;
+    SV *sv = *PL_stack_sp;
 
     sv = S_rv2gv(aTHX_
           sv, PL_op->op_private & OPpDEREF,
@@ -198,10 +199,11 @@ PP_wrapped(pp_rv2gv, 1, 0)
          );
     if (PL_op->op_private & OPpLVAL_INTRO)
         save_gp(MUTABLE_GV(sv), !(PL_op->op_flags & OPf_SPECIAL));
-    SETs(sv);
-    RETURN;
+    rpp_replace_1_1(sv);
+    return NORMAL;
 }
 
+
 /* Helper function for pp_rv2sv and pp_rv2av/hv.
  *
  * Return a GV based on the value of sv, using symbolic references etc.