Do not use IoRead or IoError in public API

[dtolnay]

The rustdoc for BincodeRead currently looks like:

pub trait BincodeRead<'storage>: IoRead { }

IoRead is a renamed import that you use internally. Since this is user-facing, it should use the standard names for std types.

[dtolnay]

Same thing for IoError.

pub enum ErrorKind {
    IoError(IoError),
    InvalidEncoding {
        desc: &'static str,
        detail: Option<String>,
    },
    SizeLimit,
    SequenceMustHaveLength,
    Custom(String),
}