Diagnostics quality of life improvements (#2375)

* Implement `Debug` manually for CandidateHash

This will make candidate hashes printed consistently without the
`CandidateHash(` and `)` decorations.

* Do not print CompressedPov's guts

It can be overwhelming. Better just use the size.

* Log when candidate is generated

* Print para_id and candidate_hash upon receiving a collation

polkadot/node/collation-generation/src/lib.rs

@@ -337,6 +337,14 @@ async fn handle_new_activations<Context: SubsystemContext>(
 					},
 				};
 
+				tracing::debug!(
+					target: LOG_TARGET,
+					candidate_hash = %ccr.hash(),
+					?pov_hash,
+					?relay_parent,
+					para_id = %scheduled_core.para_id,
+					"candidate is generated",
+				);
 				metrics.on_collation_generated();
 
 				if let Err(err) = task_sender.send(AllMessages::CollatorProtocol(