Implement Error for CoseError

[janosimas]

Is there any reason for CoseError not to implement Error?
This causes issues when working with other libraries, like thiserror

[daviddrysdale]

The original reason was because this crate is #[no_std], and Error used to be std-only.

AIUI Error has now moved into core, but I guess that would require a suitable MSRV to use.

In the meanwhile, I guess we could add a (non-default) std feature that adds an Error implementation.

[janosimas]

The std feature looks good to me

[daviddrysdale]

Fixed in #61