`TextViewBuffer` does not correctly parse macros in structs

[popematt]

Here are the inputs that are currently failing to parse. I did some investigation, and it seems like it is parsing the content just fine, but it is failing somewhere in the raw struct reader/iterator.

    use rstest::rstest;
    #[rstest]
    #[case::e_exp_in_struct("{(:foo)}")]
    #[case::e_exp_in_struct_with_space_before("{ (:foo)}")]                       // FAILS
    #[case::e_exp_in_struct_with_space_after("{(:foo) }")]
    #[case::e_exp_in_struct_field("{a:(:foo)}")]                                  // FAILS
    #[case::e_exp_in_struct_field_with_comma("{a:(:foo),}")]                      // FAILS
    #[case::e_exp_in_struct_field_with_comma_and_second_field("{a:(:foo), b:2}")]
    #[case::e_exp_in_struct_field_with_space_before("{ a:(:foo)}")]               // FAILS
    #[case::e_exp_in_struct_field_with_space_after("{a:(:foo) }")]                // FAILS
    fn test_match_macro_invocation_in_context(#[case] input: &str) {
        MatchTest::new(input).expect_match(match_length(TextBufferView::match_top_level_item_1_1));
    }