Improve io and error

[Pratyush]

This makes the API match that of std better.

[burdges]

I'll remark that (1) https://github.com/dataphract/acid_io exists and that (2) small box, or more tiny box, optimizations are both doable in Rust and permit roughly pub type IOErrorType = TinyBox<dyn ErrorTrait> where ErrorTrait removes BackTrace rust-lang/rust#48331 (comment) although doing so disrupts niches under some particular scenario.

Nothing to worry about right now. :)