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/V3Expand.cpp

@@ -905,7 +905,6 @@ public:
     }
     virtual ~ExpandVisitor() {}
     void main(AstNode* nodep) {
-	AstNode::userClearTree();	// userp() used on entire tree
 	nodep->accept(*this);
     }
 };