Internals: Refactor MT scheduling (#8012)

Prep for fixing test added in #7913.

This is a large scale no functional change refactor, however, MT output
is perturbed as tied scores will be broken differently due to ordering
changes (still deterministic).

Split multi-threaded scheduling out of the monolithic
V3OrderParallel.cpp, into relatively independent parts, simplify the
data structures, and drop redundant or unused code.

New translation units:
- V3OrderMTaskGraph.h/.cpp: OrderMTaskGraph, the graph of LogicMTask
  vertices and MTaskEdge edges. LogicMTask and MTaskEdge no longer
  depend on the coarsening algorithm's merge candidate types;
  per-algorithm auxiliary data is attached externally via the vertex and
  edge user pointers.
- V3OrderMTaskFixHazards.cpp: data hazard fixup, was FixDataHazards.
- V3OrderMTaskContraction.cpp: graph coarsening, was Partitioner
  together with PropagateCp and the merge candidate types.
- V3OrderParallel.cpp: now just the partitioning driver and ExecMTask
  graph construction.

Data structure changes:
- Delete V3Scoreboard.h/.cpp. The generic template had a single user, now
  a file-local MergeCandidateScoreboard in V3OrderMTaskContraction.cpp.
- Merge candidates are now MergeCandidate/SiblingMC/EdgeMC, distinguished
  by a bit in the candidate id rather than by a vtable, and allocated by
  the scoreboard, which owns their lifetime. This removes the multiple
  inheritance previously used by MTaskEdge.

Move `hashGraphDebug` which prints the hash of a graph's shape for debugging
to generic `V3Graph::hashGraphDebug`.

Removed (can be added back later):
- Unnecesasry self tests that force special data stucture requirements.
- Per stage --stats output under --debug. (Final figures still reported.)
- Various debug dumps

src/V3OrderMoveGraph.h

@@ -82,11 +82,21 @@ public:
 // OrderMoveGraph is constructed from the fine-grained OrderGraph.
 // It is a slightly coarsened representation of dependencies used to drive serialization.
 class OrderMoveGraph final : public V3Graph {
+    OrderGraph& m_orderGraph;  // The OrderGraph this move graph was built from
+
 public:
+    explicit OrderMoveGraph(OrderGraph& orderGraph)
+        : m_orderGraph{orderGraph} {}
+
+    OrderGraph& orderGraph() const { return m_orderGraph; }
+
     // Build an OrderMoveGraph from an OrderGraph
     static std::unique_ptr<OrderMoveGraph> build(OrderGraph&, const V3Order::TrigToSenMap&);
 };
 
+//======================================================================
+// OrderMoveDomScope
+
 // Information stored for each unique (domain, scope) pair. Mainly a list of ready vertices under
 // that (domain, scope). OrderMoveDomScope instances are themselves organized into a global ready
 // list if they have ready vertices.