Implement contains_key for PcdCommitments + change keys() return value

src/pcd.rs

@@ -234,6 +234,10 @@ impl PcdCommitments {
         Ok(())
     }
 
+    pub fn contains_key(&self, key: &str) -> bool {
+        self.0.contains_key(key)
+    }
+
     pub fn get(&self, field: &str) -> Option<&[u8; 32]> {
         self.0.get(field)
     } 
@@ -250,8 +254,8 @@ impl PcdCommitments {
         self.0.iter_mut()
     }
 
-    pub fn keys(&self) -> Vec<String> {
-        self.0.keys().map(|k| k.to_owned()).collect()
+    pub fn keys(&self) -> Keys<String, [u8; 32]> {
+        self.0.keys()
     }
 
     /// Since BTreeMap keys order is deterministic, we can guarantee a consistent merkle tree