Implement `tracing::Value` for `Slot`/`Epoch`

[michaelsproul]

We quite often want to log slots and epochs.

At the moment we need to use %slot to log them with tracing (to use the Display impl). I think it would simplify things a little if we implemented tracing::Value for Slot and Epoch so that the % prefix isn't required.

[varun-doshi]

Please correct me if Im wrong but it seems like Value requires Sealed which is not allowed for any custom types.
ref: tokio-rs/tracing#1455

[michaelsproul]

@varun-doshi Oh that's interesting, I didn't realise. I think you're right, we'd need to wait until they open up the Value trait.

I'll mark this issue as blocked, but maybe we should just close it

[varun-doshi]

Yeah doesn't seem like they plan on opening up Sealed to custom types anytime soon