Support vpiModule, bug1469.

Add very basic support for vpiModule. Basically it allows to traverse
the module tree to find a variable etc. It does not support more than
vpi_iterate and vpi_scan for vpiModule along basic operations like
vpi_get_str on vpiModule.

The support is added non-intrusively to non-VPI verilator runs. It
essentially:

 - Tracks the creation of cell instances and keeps them alive until
   the emit phase. They are there converted to scopes if modules.

 - Emits empty (don't add anything during construction)
   VerilatedScopes for all inlined modules, only for those inlined
   modules that are on the hierarchical path to public variables.

 - Adds VerilatedHierarchy as abstraction to structure of the
   scopes. It is only created for VPI designs. It allows to traverse
   the hierarchy from the top (NULL).

Signed-off-by: Stefan Wallentowitz <[email protected]>
Signed-off-by: Wilson Snyder <[email protected]>

src/V3Scope.cpp

@@ -151,6 +151,9 @@ private:
 
         // ***Note m_scopep is passed back to the caller of the routine (above)
     }
+    virtual void visit(AstCellInline* nodep) {
+        nodep->scopep(m_scopep);
+    }
     virtual void visit(AstActive* nodep) {
         nodep->v3fatalSrc("Actives now made after scoping");
     }