Update Rust nightly

Supersedes #368.

Adds exceptions for unwrap_or_default due to preference against Default's
ambiguity.

coordinator/tributary/src/provided.rs

@@ -77,6 +77,7 @@ impl<D: Db, T: Transaction> ProvidedTransactions<D, T> {
     }
 
     let current_provided_key = self.current_provided_key();
+    #[allow(clippy::unwrap_or_default)]
     let mut currently_provided = self.db.get(&current_provided_key).unwrap_or(vec![]);
 
     let mut txn = self.db.txn();