rename Pollard to MemForest

What we call Pollard is just the full forest with all leaves in it. A
Pollard should give the option to only cache a subset of the leaves in
the accumulator. This commit simply creates a new mod called MemForest
where the old code will live.

examples/full-accumulator.rs

@@ -5,7 +5,7 @@
 use std::str::FromStr;
 
 use rustreexo::accumulator::node_hash::NodeHash;
-use rustreexo::accumulator::pollard::Pollard;
+use rustreexo::accumulator::mem_forest::MemForest;
 use rustreexo::accumulator::proof::Proof;
 use rustreexo::accumulator::stump::Stump;
 
@@ -16,26 +16,26 @@ fn main() {
         NodeHash::from_str("d3bd63d53c5a70050a28612a2f4b2019f40951a653ae70736d93745efb1124fa")
             .unwrap(),
     ];
-    // Create a new Pollard, and add the utxos to it
-    let mut p = Pollard::new();
+    // Create a new MemForest, and add the utxos to it
+    let mut p = MemForest::new();
     p.modify(&elements, &[]).unwrap();
 
-    // Create a proof that the first utxo is in the Pollard
+    // Create a proof that the first utxo is in the MemForest
     let proof = p.prove(&[elements[0]]).unwrap();
     // Verify the proof. Notice how we use the del_hashes returned by `prove` here.
     let s = Stump::new()
         .modify(&elements, &[], &Proof::default())
         .unwrap()
         .0;
     assert_eq!(s.verify(&proof, &[elements[0]]), Ok(true));
-    // Now we want to update the Pollard, by removing the first utxo, and adding a new one.
+    // Now we want to update the MemForest, by removing the first utxo, and adding a new one.
     // This would be in case we received a new block with a transaction spending the first utxo,
     // and creating a new one.
     let new_utxo =
         NodeHash::from_str("cac74661f4944e6e1fed35df40da951c6e151e7b0c8d65c3ee37d6dfd3bc3ef7")
             .unwrap();
     p.modify(&[new_utxo], &[elements[0]]).unwrap();
 
-    // Now we can prove that the new utxo is in the Pollard.
+    // Now we can prove that the new utxo is in the MemForest.
     let _ = p.prove(&[new_utxo]).unwrap();
 }