Insert levels optimize number of inner states

[koniksedy]

This PR includes:

• Removal of the default_symbol parameter from the mata::nft::insert_levels() function, as it seems to have no use. This change improves the readability of the code.
• Modifications to the mata::nft::insert_levels() function to create only the important inner states when inserting new levels, and to utilize jump transitions.
• Improvements to the mata::nft::Nft::insert_identity() method to create an identity utilizing one self-loop transition when using JumpMode::RepeatSymbol.
  • TODO: Evaluate the performance difference between adding a self-lop transition and inserting a transition for each level.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.91667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 74.51%. Comparing base (73ad4f6) to head (5a94d88). Report is 3 commits behind head on states_with_levels.

Files	Patch %	Lines
src/nft/operations.cc	97.29%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## states_with_levels #397 +/- ## ====================================================== - Coverage 74.66% 74.51% -0.16% ====================================================== Files 43 43 Lines 5487 5509 +22 Branches 1215 1221 +6 ====================================================== + Hits 4097 4105 +8 - Misses 947 958 +11 - Partials 443 446 +3 ```

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