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/V3Assert.cpp

@@ -40,6 +40,7 @@ private:
     // NODE STATE/TYPES
     // Cleared on netlist
     //  AstNode::user()		-> bool.  True if processed
+    AstUserInUse	m_inuse1;
 
     // STATE
     AstModule*	m_modp;		// Last module
@@ -288,7 +289,6 @@ public:
 	m_beginp = NULL;
 	m_modp = NULL;
 	// Process
-	AstNode::userClearTree();	// userp() used on entire tree
 	nodep->accept(*this);
     }
     virtual ~AssertVisitor() {