This expands the core::error::Error implementations for both embedded_io::ReadExactError and embedded_io::WriteFmtError to implement the source() function to return the underlying error contained in the Other variant.

Note that this changes the bounds for the core::error::Error implementations to both types to require E: core::error::Error + 'static, but in practice this shouldn't cause many issues since using both Read::read_exact() and Write::write_fmt() relies on the ErrorType trait, which requires its Error to implement core::error::Error. Technically this is a breaking change, and would possibly break places where someone is constructing these error types outside of the context of the io traits.