Internal coding: Assert that user#() don't overlap.

Any use of a user() must now be declared, generally in the Visitor class with
    AstUser#InUse   arbitrary_object
This lets the code track if there's another request for the same user(),
preventing nasty hard to debug cases where they overlap.  This will also
call user#ClearTree(), so a bunch of those were removed, though many
extranious ones still remain.

src/V3Cast.cpp

@@ -60,6 +60,7 @@ private:
     // NODE STATE
     // Entire netlist:
     //   AstNode::user()		// bool.  Indicates node is of known size
+    AstUserInUse	m_inuse1;
 
     // STATE
     //int debug() { return 9; }
@@ -171,7 +172,6 @@ private:
 public:
     // CONSTUCTORS
     CastVisitor(AstNetlist* nodep) {
-	AstNode::userClearTree();
 	nodep->accept(*this);
     }
     virtual ~CastVisitor() {}