feat(rln): improve the PmTreeConfig initialization process with builder pattern (#334)

utils/src/merkle_tree/error.rs

@@ -26,6 +26,8 @@ pub enum ZerokitMerkleTreeError {
 pub enum FromConfigError {
     #[error("Error while reading pmtree config: {0}")]
     JsonError(#[from] serde_json::Error),
+    #[error("Error while creating pmtree config: missing path")]
+    MissingPath,
     #[error("Error while creating pmtree config: path already exists")]
     PathExists,
 }

utils/src/merkle_tree/optimal_merkle_tree.rs

@@ -296,7 +296,7 @@ where
         *self
             .nodes
             .get(&(depth, index))
-            .unwrap_or_else(|| &self.cached_nodes[depth])
+            .unwrap_or(&self.cached_nodes[depth])
     }
 
     /// Computes the hash of a node’s two children at the given depth.