Better handle nesting of scopes inside generate blocks.

Within generate schemes it is possible to have nested scopes, even
more liberally then outside generate blocks. So see to it that the
scopes properly stack with the generate blocks, and that wires and
behaviors are put in the right scopes.

PScope.h

@@ -38,7 +38,22 @@ class NetScope;
  * NOTE: This is note the same concept as the "scope" of an elaborated
  * hierarchy. That is represented by NetScope objects after elaboration.
  */
-class PScope {
+
+class LexicalScope {
+
+    public:
+      explicit LexicalScope()  { }
+	// A virtual destructor is so that dynamic_cast can work.
+      virtual ~LexicalScope() { }
+
+	// Nets an variables (wires) in the scope
+      map<perm_string,PWire*>wires;
+      PWire* wires_find(perm_string name);
+
+    private:
+};
+
+class PScope : public LexicalScope {
 
     public:
 	// When created, a scope has a name and a parent. The name is
@@ -56,10 +71,6 @@ class PScope {
       perm_string pscope_name() const { return name_; }
       PScope* pscope_parent() { return parent_; }
 
-	// Nets an variables (wires) in the scope
-      map<perm_string,PWire*>wires;
-      PWire* wires_find(perm_string name);
-
 	// Named events in the scope.
       map<perm_string,PEvent*>events;