fix for issue #4; Memory leak of NsfMethodContext with Tcl9

Ths is not a fully satisfying fix, since it just fixes the symptom, not the cause.
Also the old variant is supposed to run free of this issue. However, we are
talking here about a single Tcl_Obj inside the nx object system, so the harm
is very limited. More details in the ticket.

library/nx/nx.tcl

@@ -2500,7 +2500,7 @@ namespace eval ::nx {
     }
     set traceSpec [expr {[info exists trace] ? [list -trace $trace] : ""}]
 
-    set r [[self] object variable \
+    set r [:object variable \
                -accessor $accessor \
                -incremental=$incremental \
                -class $class \