Forward all default methods for I/O impls

[thaliaarchi]

Forward all default methods for &mut T and Box<T> to the inner io::Read, io::Write, io::Seek, and io::BufRead types.

[rustbot]

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[workingjubilee]

These forwarding impls are uncontroversial by definition and the specifics of how we implement them can be changed at our leisure, so,

@bors r+ rollup

[bors]

📌 Commit 11c1740 has been approved by workingjubilee

It is now in the queue for this repository.