Use the same serial ordering within MTasks as we use in serial mode (#4994)

The goal here is to use as single ordering heuristic (which can be
improved later) within MTasks as we do for serial code ordering. The
heuristic itself is factored out into the new OrderMoveGraphSerializer.
This also yields slightly nicer ordering than the previously use
GraphStream, so we end up with fewer trigger (domain) conditionals in
the MTasks, this can be worth a few percent speedup.

This has the somewhat nice side-effect of reusing OrderMoveGraphVertex
for both serial and parallel mode, so MTaskMoveGraphVertex can be
removed.

Serial mode yields identical output.

src/V3ExecGraph.cpp

@@ -39,6 +39,8 @@ ExecMTask::ExecMTask(V3Graph* graphp, AstMTaskBody* bodyp) VL_MT_DISABLED  //
       m_id{s_nextId++},
       m_hashName{V3Hasher::uncachedHash(bodyp).toString()} {
     UASSERT_OBJ(bodyp->stmtsp(), bodyp, "AstMTaskBody should already be populated for hashing");
+    UASSERT_OBJ(!bodyp->execMTaskp(), bodyp, "AstMTaskBody already linked to an ExecMTask");
+    bodyp->execMTaskp(this);
 }
 
 void ExecMTask::dump(std::ostream& str) const {