ES6 symbols: Implement Symbol intrinsic and basic functionality

src/macros.py

Index: src/macros.py
diff --git a/src/macros.py b/src/macros.py
index f871fc55c7358afb289ec886be0e2be140cc8c9f..291a898f53d22b26a15b4bdf61deea00dd4e8149 100644
--- a/src/macros.py
+++ b/src/macros.py
@@ -99,6 +99,7 @@ macro IS_UNDEFINED(arg)         = (typeof(arg) === 'undefined');
 macro IS_NUMBER(arg)            = (typeof(arg) === 'number');
 macro IS_STRING(arg)            = (typeof(arg) === 'string');
 macro IS_BOOLEAN(arg)           = (typeof(arg) === 'boolean');
+macro IS_SYMBOL(arg)            = (%_IsSymbol(arg));
 macro IS_OBJECT(arg)            = (%_IsObject(arg));
 macro IS_ARRAY(arg)             = (%_IsArray(arg));
 macro IS_FUNCTION(arg)          = (%_IsFunction(arg));