initial support for nil, make-list

unfinished, but usable.. still 0 is false in libs and code

ml.h

@@ -98,6 +98,9 @@ public:
     // Construct a string
     static MlValue string(const std::string &s);
 
+    // Construct a nil
+    static MlValue nil();
+
     // Construct a lambda function
     MlValue(const std::vector<MlValue> &params, MlValue ret, MlEnvironment const &env);
 
@@ -197,7 +200,8 @@ private:
 	STRING,
 	LAMBDA,
 	BUILTIN,
-	UNIT
+	UNIT,
+	NIL
     } type;
 
     union {