The hadnling of `io::Error` in sighash had a few problems:
* It used `io::ErrorKind` instead of `io::Error` losing inforation
* Changing `io::ErrorKind` to `io::Error` would disable `PartialEq`&co
* The `Io` error wariants were duplicated
It turns out all of these can be solved by moving the `Io` variant into
a separate error.
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