message: Simplify short array copies
diff: @@ -1269,7 +1269,7 @@ pub extern "system" fn librustzcash_mmr_append(
  .root_node()
  .expect("Just added, should resolve always; qed");
  unsafe {
  - (*rt_ret).copy_from_slice(&root_node.data().subtree_commitment);
  + *rt_ret = root_node.data().subtree_commitment;
  for (idx, next_buf) in slice::from_raw_parts_mut(buf_ret, return_count as usize)
  .iter_mut()
  @@ -1318,13 +1318,11 @@ pub extern "system" fn librustzcash_mmr_delete(
  };
  unsafe {
  - (*rt_ret).copy_from_slice(
  - &tree
  + *rt_ret = tree
  .root_node()
  .expect("Just generated without errors, root should be resolving")
  .data()
  - .subtree_commitment,
  - );
  + .subtree_commitment;
  }
  truncate_len
  @@ -1349,7 +1347,7 @@ pub extern "system" fn librustzcash_mmr_hash_node(
  };
  unsafe {
  - (*h_ret).copy_from_slice(&node.hash()[..]);
  + *h_ret = node.hash();
  }
  0
  