Fix strange slice GC issue

Looks like allocating the BoxedSlice before the arguments caused a issue?!?

src/runtime/capi.cpp

@@ -447,7 +447,7 @@ extern "C" PyObject* PySequence_GetItem(PyObject* o, Py_ssize_t i) noexcept {
 extern "C" PyObject* PySequence_GetSlice(PyObject* o, Py_ssize_t i1, Py_ssize_t i2) noexcept {
     try {
         // Not sure if this is really the same:
-        return getitem(o, new BoxedSlice(boxInt(i1), boxInt(i2), None));
+        return getitem(o, createSlice(boxInt(i1), boxInt(i2), None));
     } catch (ExcInfo e) {
         Py_FatalError("unimplemented");
     }

src/runtime/list.cpp

@@ -785,10 +785,11 @@ extern "C" int PyList_SetSlice(PyObject* a, Py_ssize_t ilow, Py_ssize_t ihigh, P
     ASSERT(isSubclass(l->cls, list_cls), "%s", l->cls->tp_name);
 
     try {
+        BoxedSlice* slice = (BoxedSlice*)createSlice(boxInt(ilow), boxInt(ihigh), None);
         if (v)
-            listSetitemSlice(l, new BoxedSlice(boxInt(ilow), boxInt(ihigh), None), v);
+            listSetitemSlice(l, slice, v);
         else
-            listDelitemSlice(l, new BoxedSlice(boxInt(ilow), boxInt(ihigh), None));
+            listDelitemSlice(l, slice);
         return 0;
     } catch (ExcInfo e) {
         setCAPIException(e);