use std::string_view in string_utils

[kroening]

This introduces std::string_view as a replacement for std::string parameters for the string utility functions in util/string_utils.h, in the hope of avoiding copies of string contents.

The change is designed not to break any usage of the functions; C++17 guarantees that there's an implicit conversion from std::string to std::string_view.

• 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 98.55072% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.55%. Comparing base (166a7d4) to head (e81b174).

Files with missing lines	Patch %	Lines
src/util/string_utils.cpp	78.94%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9005   +/-   ##
========================================
  Coverage    80.55%   80.55%           
========================================
  Files         1707     1707           
  Lines       189016   189015    -1     
  Branches        73       73           
========================================
+ Hits        152261   152262    +1     
+ Misses       36755    36753    -2     

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