bugfix: 'node_16<T>::grow()' partial_key index offset inconsistent

[bowling233]

This issue was discovered when using the iterator to traverse and output text.

The functions in node_48.hpp will all add an offset of 128 when indexing using partial_key, for example:

node_48<T>::find_child(char partial_key){
    uint8_t index = indexes_[128 + partial_key];
    //...
}

However, node_16<T>::grow() ignores this point and directly uses the partial_key stored in keys_ as the index, causing an offset in indexing when the node grows:

template <class T> inner_node<T> *node_16<T>::grow() {
  //...
  for (int i = 0; i < n_children_; ++i) {
    new_node->indexes_[(uint8_t) this->keys_[i]] = i;
  }  //...
}

For example, consider a node_16 instance:

node_16:
keys_: [a, b, ...]
ASCII: [97, 98, ...]
node_16<T>::grow()
indexes_: [..., 0, ..., EMPTY, ...]
        wrong [97]  correct[97+128]

When querying a in node_48, find_child() will index indexes_[128 + 97], but the value there is EMPTY. Moreover, this error will also affect the correctness of functions like node_48<T>::next_partial_key, causing iterators to obtain incorrect values of keys_, and so on.

The iterator in the figure obtained an incorrect value of -55 through node_48<T>::next_partial_key. This node was grown from a node_16, where the original key stored at that position was I (73). node_48<T>::next_partial_key(-128) start from -128+128 with an offset of 73 to get I, but partial_key remains -128+73=-55.