Cache `num_leaves` / `num_entries` in `LargeSmt`

[krushimir]

Currently methods like:

pub fn num_leaves(&self) -> Result<usize, LargeSmtError> {
    Ok(self.storage.leaf_count()?)
}

delegate to storage and must return a Result.

It might make sense to cache values such as num_leaves and num_entries directly in LargeSmt.

From discussion in PR #438.