Implement Clone, Eq and PartialEq for Error.

[qwandor]

This fixes #217.

[Keats]

Can I just get more details on when you would use eq on the error instead of on the error kind? And when is PartialEq not enough?

[qwandor]

I'd like to be able to #[derive(Clone, Eq, PartialEq)] my own error types which wrap jsonwebtoken::Error. In particular I have something like this:

#[error(Debug, Clone, Eq, PartialEq, thiserror::Error)]
pub enum MyError {
    #[error("JWT error: {0}")]
    Jwt(jsonwebtokenerror::Error),
    #[error("Some other error...")]
    SomeOtherError(...),
    ...
}

For the derive macros to work, the various wrapped error types need to implement Clone and Eq themselves. All the error types provided by other libraries I use do, it's just jsonwebtoken which doesn't.

As for PartialEq vs. Eq, I guess PartialEq is 'enough' in that it implements all the methods I need, but there's no reason not to implement Eq and it's correct to do so. It's just a marker trait which says that your PartialEq implementation is reflexive, symmetric and transitive, which it already is.