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/V3Dead.cpp

@@ -70,6 +70,7 @@ private:
     //  AstModule::user()	-> int. Count of number of cells referencing this module.
     //  AstVar::user()		-> int. Count of number of references
     //  AstVarScope::user()	-> int. Count of number of references
+    AstUserInUse	m_inuse1;
 
     // TYPES
     typedef multimap<AstVarScope*,AstNodeAssign*>	AssignMap;
@@ -188,7 +189,6 @@ public:
 	m_elimUserVars = elimUserVars;
 	m_sideEffect = false;
 	// Operate on whole netlist
-	AstNode::userClearTree();	// userp() used on entire tree
 	nodep->accept(*this);
 	deadCheckVar();
 	// Modules after vars, because might be vars we delete inside a mod we delete