use std::string_view for unicode.h functions

[kroening]

Many of the functions only 'view' the string given, and hence, it the use of std::string_view may relieve the caller of the burden of constructing the std::string object.

• 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

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.78%. Comparing base (1e835df) to head (27aad27).

Files with missing lines	Patch %	Lines
src/util/unicode.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9100      +/-   ##
===========================================
+ Coverage    80.77%   80.78%   +0.01%     
===========================================
  Files         1712     1712              
  Lines       189814   189814              
  Branches        73       73              
===========================================
+ Hits        153324   153344      +20     
+ Misses       36490    36470      -20     

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