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/V3Combine.cpp

@@ -174,6 +174,9 @@ private:
     //  AstCFunc::user3p()	-> AstCFunc*, If set, replace ccalls to this func with new func
     //  AstNodeStmt::user3()	-> AstNode*.  True if to ignore this cell
     //  AstNodeStmt::user4()	-> V3Hashed::V3Hash.  Hash value of this node (hash of 0 is illegal)
+    AstUserInUse	m_inuse1;
+    AstUser3InUse	m_inuse3;
+    //AstUser4InUse	part of V3Hashed
 
     // STATE
     typedef enum {STATE_IDLE, STATE_HASH, STATE_DUP} CombineState;