io: require Fron<ErrorKind> for Error

[Conaclos]

Fix #688

This PR requires the implementation of Fron<ErrorKind> for Error.
This allows several simplifications in error handling and make embedded-io closer to std::io.
Notably:

• This allows to remove ReadExactError and to replace it with ErrorKind::UnexpectedEof. This is what std::io does.
• This allows to remove SliceMutError and to replace it with ErrorKind::StorageFull.

The PR also removes the implementation of Error for Infallible. std::io doesn't allow this.