Internals: Replace VlTriggerVec with unpacked array (#6616)

Removed the VlTriggerVec type, and refactored to use an unpacked array
of 64-bit words instead. This means the trigger vector and its
operations are now the same as for any other unpacked array. The few
special functions required for operating on a trigger vector are now
generated in V3SchedTrigger as regular AstCFunc if needed.

No functional change intended, performance should be the same.

src/V3OrderGraphBuilder.cpp

@@ -136,7 +136,7 @@ class OrderGraphBuilder final : public VNVisitor {
         VL_RESTORER(m_hybridp);
         VL_RESTORER(m_inClocked);
 
-        // This is the original sensitivity of the block (i.e.: not the ref into the TRIGGERVEC)
+        // This is the original sensitivity of the block (i.e.: not the ref into the trigger vec)
 
         const AstSenTree* const senTreep = nodep->sentreep()->hasCombo()
                                                ? nodep->sentreep()