Improve insert_word and insert_identity

[koniksedy]

This PR, based on notes from PR-Nfa compose, includes:

• Modifications to methods nfa::Nfa::insert_word(), nft::Nft::insert_word(), and nft::Nft::insert_word_by_parts(), which now have an optional target state (default Limits::max_state).
  • If the target state is not specified, a new state will be created.
  • These methods return the target state.
• An overloaded method nft::Nft::insert_identity() that can create identity transitions for each symbol in a given vector.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.43%. Comparing base (485b91e) to head (f0ac876). Report is 1 commits behind head on states_with_levels.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## states_with_levels #394 +/- ## ====================================================== + Coverage 73.32% 73.43% +0.10% ====================================================== Files 43 43 Lines 5170 5180 +10 Branches 1173 1173 ====================================================== + Hits 3791 3804 +13 + Misses 942 939 -3 Partials 437 437 ```

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[Adda0]

We cannot use Limits::max_state. We need to probably have two overloaded functions. One with the normal parameters without target state which will inside the function default to nfa.num_of_states(), and another with explicit target state parameter (the original function). I consider this approach a cleaner solution than the current one since Limits::max_state is still technically a valid state.