Add a symbol __curPos that expands to the current source location

I.e. an attribute set { file = <string>; line = <int>; column = <int>; }.

src/libexpr/nixexpr.cc

@@ -130,6 +130,11 @@ void ExprConcatStrings::show(std::ostream & str)
     }
 }
 
+void ExprPos::show(std::ostream & str)
+{
+    str << "__curPos";
+}
+
 
 std::ostream & operator << (std::ostream & str, const Pos & pos)
 {
@@ -315,6 +320,10 @@ void ExprConcatStrings::bindVars(const StaticEnv & env)
         (*i)->bindVars(env);
 }
 
+void ExprPos::bindVars(const StaticEnv & env)
+{
+}
+
 
 /* Storing function names. */