core::error::Error eixsts now, so ark-std should switch over. I've no idea if this changes anything else behind the scenes.

core::io 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 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 #44