Optimize Subset Construction using hash

[AnicoderAndy]

In the original implementation, the function convert_completed in dfa/construction.py traverses the whole dfa_list, which leads to a time complexity of $O(nm)$ in the worst case, where $n$ is the size of nfa_closure and $m$ is the size of dfa_list. By introducing hash, time complexity of this procedure decreases to $O(n)$. This may help improve efficiency when inputs are complicated.