Implement Display for Event, thread through "display()" function for Envelope

As it says on the tin! Implement `Display` for [`Event`](https://github.com/entropic-dev/eos-spike/blob/ab29b8ac64dac9754416702c0f643f3ce5a62032/src/objects/event.rs#L202) for pretty-printing, and add a new method to [`Envelope`](https://github.com/entropic-dev/eos-spike/blob/ab29b8ac64dac9754416702c0f643f3ce5a62032/src/envelope.rs#L20) to expose that display (or change the [current `Display` impl](https://github.com/entropic-dev/eos-spike/blob/ab29b8ac64dac9754416702c0f643f3ce5a62032/src/envelope.rs#L10), but make sure to change the `self.to_string()` call here if you do!)