Adjust usage of `std::io::ErrorKind` to be `core` compatible

* Checking exhaustion will no longer be possible for `repr_bitpacked`. Moving `kind_from_prim` into an associated function, and setting it up to be moved into `core::io` as well.
* `ErrorKind::as_str` is private, but it's only usage is trivially replaced with `Display::fmt`
* The features io_error_inprogress, io_error_more, and io_error_uncategorized will all need to be enabled