core::error::Error

[core::error::Error](https://doc.rust-lang.org/nightly/core/error/trait.Error.html) eixsts now, so ark-std should switch over. I've no idea if this changes anything else behind the scenes.

[core::io](https://doc.rust-lang.org/nightly/core/io/index.html) exists too, but only experimental, and obnoxiously thin, so pretty useless. There are anyways good reasons to re-fork `std::io`, and especially `std::io::error`, like `ErrorKind` being `Clone`, but..

At present, art-std is the single most popular non-std `::io`, but together [embedded-io and embedded-io-async](https://github.com/rust-embedded/embedded-hal) have slightly more dependents on crates.io, although not as many on github. We could make "abdicate" maintaining this by switching `ark_std::io` over to embedded-io or embedded-io-async?. Nope, embedded-io and embedded-io-async lack `Cursor` but maybe some other non-std `::io` wins up more fully featured.

Superceeds https://github.com/arkworks-rs/std/issues/44

Start by comparing ark_std::io and the linked core::error::Error API, then review the existing issue 44 context and the embedded-io alternatives mentioned here. Determine the compatibility and downstream impact of switching ark-std to core::error::Error; done means the replacement is agreed and the related behavior remains supported.

Written by the indexing model from the issue text.