feat: partial pollard

Partial pollards allow you to hold only a subset of utxos in the forest,
effectively caching them and not requiring to download or validate extra
proofs for it.

This commit implements it using a very similar approach as the main
forest, but with the possibility to prune leaves that you don't want to
hold anymore.

examples/full-accumulator.rs

@@ -4,8 +4,8 @@
 
 use std::str::FromStr;
 
-use rustreexo::accumulator::node_hash::NodeHash;
 use rustreexo::accumulator::mem_forest::MemForest;
+use rustreexo::accumulator::node_hash::NodeHash;
 use rustreexo::accumulator::proof::Proof;
 use rustreexo::accumulator::stump::Stump;
 

src/accumulator/mem_forest.rs

@@ -4,8 +4,8 @@
 //!
 //! # Example
 //! ```
-//! use rustreexo::accumulator::node_hash::NodeHash;
 //! use rustreexo::accumulator::mem_forest::MemForest;
+//! use rustreexo::accumulator::node_hash::NodeHash;
 //! let values = vec![0, 1, 2, 3, 4, 5, 6, 7];
 //! let hashes: Vec<NodeHash> = values
 //!     .into_iter()
@@ -266,8 +266,8 @@ impl MemForest {
     /// and the hashes that we what to prove, but sorted by position in the tree.
     /// # Example
     /// ```
-    /// use rustreexo::accumulator::node_hash::NodeHash;
     /// use rustreexo::accumulator::mem_forest::MemForest;
+    /// use rustreexo::accumulator::node_hash::NodeHash;
     /// let mut mem_forest = MemForest::new();
     /// let hashes = vec![0, 1, 2, 3, 4, 5, 6, 7]
     ///     .iter()
@@ -308,8 +308,8 @@ impl MemForest {
     /// use bitcoin_hashes::sha256::Hash as Data;
     /// use bitcoin_hashes::Hash;
     /// use bitcoin_hashes::HashEngine;
-    /// use rustreexo::accumulator::node_hash::NodeHash;
     /// use rustreexo::accumulator::mem_forest::MemForest;
+    /// use rustreexo::accumulator::node_hash::NodeHash;
     /// let values = vec![0, 1, 2, 3, 4, 5, 6, 7];
     /// let hashes = values
     ///     .into_iter()
@@ -633,8 +633,8 @@ mod test {
     use serde::Deserialize;
 
     use super::MemForest;
-    use crate::accumulator::node_hash::NodeHash;
     use crate::accumulator::mem_forest::Node;
+    use crate::accumulator::node_hash::NodeHash;
     use crate::accumulator::proof::Proof;
 
     fn hash_from_u8(value: u8) -> NodeHash {

src/accumulator/mod.rs

@@ -7,8 +7,9 @@
 //! a lightweight implementation of utreexo that only stores roots. Although the [stump::Stump]
 //! only keeps the accumulator's roots, it still trustlessly update this state, not requiring
 //! a trusted third party to learn about the current state.
-pub mod node_hash;
 pub mod mem_forest;
+pub mod node_hash;
+pub mod pollard;
 pub mod proof;
 pub mod stump;
 pub(super) mod util;