Simplifying get_useful_states with move iterator

[kilohsakul]

Look at this to see an ultimate demonstration of the coolness of the davids move iterator.

Although, I am wandering whether it can be used anywhere else then here. I did not come with a good usage in the intersection for instance, because I wanted to get the vectors ot targets with the same symbol and then merge them. Maybe it is just my fault, but maybe we actually need something a bit different, I don't know.
For instance, iterator through symbol_post that skips over symbols where there are no targets ...
Just thinking.

[kilohsakul]

Just a straightforward use of the move iterator, should be no brainer.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8684c78) 71.59% compared to head (d92fe19) 71.52%.
Report is 1 commits behind head on devel.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #353      +/-   ##
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- Coverage   71.59%   71.52%   -0.07%     
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  Files          30       30              
  Lines        3622     3603      -19     
  Branches      835      830       -5     
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- Hits         2593     2577      -16     
+ Misses        737      736       -1     
+ Partials      292      290       -2     

Files	Coverage Δ
include/mata/nfa/delta.hh	90.41% <100.00%> (-0.13%)	⬇️
src/nfa/nfa.cc	80.09% <88.88%> (-0.25%)	⬇️

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[vhavlena]

nice

[Adda0]

Other than the suggested, the use of iterators is elegant and readable. We will see how many uses the iterators have. Different variants can be added as needed, if we find them useful.

[Adda0]

It seems to be just a tad bit slower (due to creating the instances of the iterators), but the difference should not be that significant. The experiments will tell.