add an ExprPrinter class, like ValuePrinter

To be used Shortly

Change-Id: I9def7975aa55f251eb8486391677771f7352d7ce

src/libexpr/print.cc

@@ -574,4 +574,10 @@ fmt_internal::HintFmt & fmt_internal::HintFmt::operator%(const ValuePrinter & va
         return *this;
 }
 
+std::ostream & operator<<(std::ostream & output, ExprPrinter const & printer)
+{
+    printer.expr.show(printer.state.symbols, output);
+    return output;
+}
+
 }