Feature unsealed_read_write should work without std

[pyfisch]

Run cargo build --no-default-features --features unsealed_read_write.

It fails with:

error[E0599]: no variant or associated item named `Io` found for type `error::ErrorCode` in the current scope
   --> src/error.rs:53:30
    |
53  |             code: ErrorCode::Io,
    |                              ^^ variant or associated item not found in `error::ErrorCode`
...
263 | pub(crate) enum ErrorCode {
    | ------------------------- variant or associated item `Io` not found here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: Could not compile `serde_cbor`.

To learn more, run the command again with --verbose.

@baloo now we know why Error::Io is needed.

[baloo]

Good catch! Sorry I missed that