Implement Unwrap() []error for Go 1.20 multi-error support

[ian-h-chamberlain]

Go 1.20 released recently, and includes support for wrapping multiple errors.

If implemented, this could probably replace WrappedErrors(), but it would be a breaking change to return a different type from Unwrap() and would also change the result of errors.Unwrap() on a retry error (as mentioned in #65). I'm not sure what the best approach for compatibility is here but it would be nice to have an API that matches up with the standard library's approach to multi-error unwrapping.