Use `?` debug formatting for block roots in beacon_chain.rs (#2890)
Use `?` debug formatting for block roots in beacon_chain.rs (#2890)

## Issue Addressed

NA

## Proposed Changes

Ensures full roots are printed, rather than shortened versions like `0x935b…d376`.

For example, it would be nice if we could do API queries based upon the roots shown in the `Beacon chain re-org` event:

```
Jan 05 12:36:52.224 WARN Beacon chain re-org                     reorg_distance: 2, new_slot: 2073184, new_head: 0x8a97…2dec, new_head_parent: 0xa985…7688, previous_slot: 2073183, previous_head: 0x935b…d376, service: beacon
Jan 05 13:35:05.832 WARN Beacon chain re-org                     reorg_distance: 1, new_slot: 2073475, new_head: 0x9207…c6b9, new_head_parent: 0xb2ce…839b, previous_slot: 2073474, previous_head: 0x8066…92f7, service: beacon
```

## Additional Info

We should eventually fix this project-wide, however this is a short-term patch.




sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse