backtrace.c: use mrb_irep references instead of struct RProc

To reduce GC burden (no mark needed). We use `mrb_irep_incref()` and
`mrb_irep_decref()` instead to track irep memory usage; ref #6161

include/mruby/internal.h

@@ -70,7 +70,7 @@ struct RBacktrace {
 struct mrb_backtrace_location {
   mrb_sym method_id;
   int32_t idx;
-  const struct RProc *proc;
+  const mrb_irep *irep;
 };
 
 /* gc */