tibble: A `tbl` may contain an external pointer via attribute `problems` set by readr

A `tbl` may contain an external pointer via attribute `problems`, e.g.

```sh
spc_tbl_ [25,000 × 3] (S3: spec_tbl_df/tbl_df/tbl/data.frame)
$ Index : num [1:25000] 1 2 3 4 5 6 7 8 9 10 ...
$ Height_Inches: num [1:25000] 65.8 71.5 69.4 68.2 67.8 ...
$ Weight_Pounds: num [1:25000] 113 136 153 142 144 ...
- attr(*, "spec")=
.. cols(
.. Index = col_double(),
.. Height_Inches = col_double(),
.. Weight_Pounds = col_double()
.. )
- attr(*, "problems")=
```