make each variable its own little stack frame. i am, howver, going to back all this stuff out because I'm getting it wrong in some subtle but segfaulty way

include/XSLT/impl/xslt_variable_impl.hpp

@@ -37,6 +37,7 @@ public:
     DOMSink sink;
     {
       RedirectOutputFrame redirect(context, sink);
+      StackFrame frame(context);
       execute_children(node, context);
     } //