Return second argument of trace! without evaluation

Evaluating second argument before calling trace! leads to unexpected
sequence of traces. Making type of the second argument Atom means
argument is evaluated after trace! prints the first argument.

lib/src/metta/runner/stdlib/debug.rs

@@ -58,7 +58,7 @@ grounded_op!(TraceOp, "trace!");
 
 impl Grounded for TraceOp {
     fn type_(&self) -> Atom {
-        Atom::expr([ARROW_SYMBOL, ATOM_TYPE_UNDEFINED, Atom::var("a"), Atom::var("a")])
+        Atom::expr([ARROW_SYMBOL, ATOM_TYPE_UNDEFINED, ATOM_TYPE_ATOM, ATOM_TYPE_ATOM])
     }
 
     fn as_execute(&self) -> Option<&dyn CustomExecute> {