Fix empty lambda args range (#35)

Previously, empty lambda arguments (e.g. `lambda: 1`) would get the
range of the entire expression, which leads to incorrect comment
placement. Now empty lambda arguments get an empty range between the
`lambda` and the `:` tokens.

parser/src/parser.rs

@@ -636,6 +636,13 @@ mod tests {
         insta::assert_debug_snapshot!(parse_ast);
     }
 
+    #[test]
+    fn test_parse_lambda_no_args() {
+        let source = "lambda: 1";
+        let parse_ast = ast::Suite::parse(source, "<test>").unwrap();
+        insta::assert_debug_snapshot!(parse_ast);
+    }
+
     #[test]
     fn test_parse_tuples() {
         let source = "a, b = 4, 5";