Move `std::io::ErrorKind` to `core::io`

Move `std::io::ErrorKind` to `core::io`
* Update `rustdoc-html` tests for the new path
* Add `core_io` feature to control stability. This replaces the use of `core_io_borrowed_buf` on the `core::io` module itself.
* Re-export `core::io::ErrorKind` in `std::io::error`