Implement PartialEq for rmp_serde errors

[makaleks]

I use custom error using enums and store rmp_serde::encode::Error and rmp_serde::decode::Error inside one of its variants. To use assert_eq! in tests I need to implement PartialEq for this types myself. These errors may worth adding #[derive(PartialEq)], may not they?

[kornelski]

These enums may contain io::Error which isn't comparable. You can implement PartialEq manually yourself, or use matches! instead of ==.

[makaleks]

@kornelski , thank you, that was surprising for me. Was not familiar with matches!