Add 'all_subs' method to EvalPMC.

This provides API compatibility with the upcoming PackFilePMC changes
and gives NQP something to target for conversion.

src/pmc/eval.pmc

@@ -366,6 +366,22 @@ Return the main sub, if any, null PMC otherwise.
         RETURN(PMC * mainsub);
     }
 
+/*
+
+=item C<all_subs()>
+
+Return an object providing indexed access to all subs in the compilation
+unit (i.e., self).  Provides compability with new packfile_pmc API.
+
+=cut
+
+*/
+
+    METHOD all_subs()
+    {
+        return(SELF);
+    }
+
 }
 
 /*