Implement Serialize/Deserialize for std::io::ErrorKind

[bddap]

ErrorKind is a simple enum so hopefully implementing ser/de would be straightforward.

One use case: structured error reporting in networked apis. The function:

fn frob() -> Result<bool, io::ErrorKind>;

could be turned into an rpc call.

[bddap]

io:ErrorKind docs

[dtolnay]

Thanks! I would prefer not to provide these impls because this seems pretty specific to your use case -- I don't recall having seen Result<T, io::ErrorKind> before. I would recommend using serialize_with or a newtype wrapper to provide the serialization logic yourself.