MVMStaticFrame does not directly ref an MVMFrame.

Thus refs_frames can be cleared. It may have referenced them at some
point in the past, but it doesn't now. Since MVMStaticFrame was the
most costly of the refs_frames things to mark, and there were a lot
of them, this makes the intergenerational roots a bunch cheaper to
handle.

src/6model/reprs/MVMStaticFrame.c

@@ -190,5 +190,5 @@ static const MVMREPROps this_repr = {
     compose,
     "MVMStaticFrame", /* name */
     MVM_REPR_ID_MVMStaticFrame,
-    1, /* refs_frames */
+    0, /* refs_frames */
 };