feat: add no-std support

closes #5

src/merkle/merkle_tree.rs

@@ -1,7 +1,7 @@
 use super::MerkleError;
 use crate::{
     hash::{merge, Digest},
-    log2, uninit_vector, Felt, FieldElement, Word,
+    log2, uninit_vector, Felt, FieldElement, Vec, Word,
 };
 use core::slice;
 
@@ -80,7 +80,7 @@ impl MerkleTree {
             return Err(MerkleError::InvalidIndex(depth, index));
         }
 
-        let pos = 2usize.pow(depth as u32) + (index as usize);
+        let pos = 2_usize.pow(depth) + (index as usize);
         Ok(self.nodes[pos])
     }
 
@@ -102,7 +102,7 @@ impl MerkleTree {
         }
 
         let mut path = Vec::with_capacity(depth as usize);
-        let mut pos = 2usize.pow(depth as u32) + (index as usize);
+        let mut pos = 2_usize.pow(depth) + (index as usize);
 
         while pos > 1 {
             path.push(self.nodes[pos ^ 1]);