use std::string_view in edit_distance

[kroening]

The strings given as arguments to the methods in levenshtein_automatont are not copied, and their address is not taken. Hence, a std::string_view suffices.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.77%. Comparing base (3163ab8) to head (c387140).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9097   +/-   ##
========================================
  Coverage    80.77%   80.77%           
========================================
  Files         1712     1712           
  Lines       189791   189814   +23     
  Branches        73       73           
========================================
+ Hits        153304   153325   +21     
- Misses       36487    36489    +2     

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