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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

@@                  Coverage Diff                   @@
##           states_with_levels     #394      +/-   ##
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+ 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.