Implement a distinct constant pool (#3013)

What previously used to be per module static constants created in
V3Table and V3Prelim are now merged globally within the whole model and
emitted as part of a separate constant pool. Members of the constant
pool are global variables which are declared lazily when used (similar to
loose methods).

src/V3EmitCSyms.cpp

@@ -278,6 +278,7 @@ class EmitCSyms final : EmitCBaseVisitor {
             if (!m_dpiHdrOnly) emitDpiImp();
         }
     }
+    virtual void visit(AstConstPool* nodep) override {}  // Ignore
     virtual void visit(AstNodeModule* nodep) override {
         nameCheck(nodep);
         VL_RESTORER(m_modp);