Function env_duration_ms_optional

crates/ffi/src/kernel.rs:2100–2106

(var: &str, default: Duration)

    Err(e) => {
      return LeanIOResult::error_string(&format!(
        "rs_env_extract: failed to read {path}: {e}"
      ));
    },
  };
  let mut slice: &[u8] = &bytes;
  let full = match IxonEnv::get(&mut slice) {
    Ok(env) => env,
    Err(e) => {
      return LeanIOResult::error_string(&format!(
        "rs_env_extract: failed to deserialize {path}: {e}"