Use Cow<[u8]> for sort keys

src/table/util.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use serde::{Deserialize, Serialize};
 
 use garage_util::data::*;
@@ -7,8 +9,8 @@ use crate::schema::*;
 #[derive(Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct EmptyKey;
 impl SortKey for EmptyKey {
-	fn sort_key(&self) -> &[u8] {
-		&[]
+	fn sort_key(&self) -> Cow<'_, [u8]> {
+		Cow::from(&[][..])
 	}
 }
 impl PartitionKey for EmptyKey {