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/V3ExecGraph.cpp

@@ -879,8 +879,9 @@ void finalizeCosts(V3Graph* execMTaskGraphp) {
     execMTaskGraphp->removeTransitiveEdges();
 
     // Record summary stats for final m_tasks graph.
-    const auto report = execMTaskGraphp->parallelismReport(
-        [](const V3GraphVertex* vtxp) { return vtxp->as<const ExecMTask>()->cost(); });
+    const auto report = execMTaskGraphp->parallelismReport([](const V3GraphVertex* vtxp) {  //
+        return vtxp->as<const ExecMTask>()->cost();
+    });
     V3Stats::addStat("MTask graph, final, critical path cost", report.criticalPathCost());
     V3Stats::addStat("MTask graph, final, total graph cost", report.totalGraphCost());
     V3Stats::addStat("MTask graph, final, mtask count", report.vertexCount());