io: `ErrorType` could be split into `Read::Error`, `Write::Error` and `Seek::Error`.

[Conaclos]

While I Implemented Read and Write on a custom type, I noticed that the errors that can happen during Read are not always the same as the errors that happen during write.
For example, if you implement a Cursor type like the one in std::io, read cannot fail, while write can fail if the storage is full.

I propose to remove ErrorType and simply add an associated type Error to Read, Write and Seek. We don't have to add one to BufRead because it is a super trait of Read. We could make ReadReady and WriteReady super traits of Read and Write respectively.

I think this makes the code more flexible and idiomatic. This also looks more consistent with the approach of mebedded-hal of favoring fine-grained errors.