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/V3Table.cpp

@@ -368,6 +368,10 @@ class TableVisitor : public TableBaseVisitor {
 private:
     // NODE STATE
     // Cleared on each always/assignw
+    AstUserInUse	m_inuse;
+    AstUser2InUse	m_inuse2;
+    AstUser3InUse	m_inuse3;
+    AstUser4InUse	m_inuse4;
 
     // STATE
     double	m_totalBytes;		// Total bytes in tables created