util: Replace std::basic_string<unsigned> with std::basic_string<char32_t>

[xokdvium]

Fixes build with libc++19 that fails with the error:

implicit instantiation of undefined template 'std::char_traits'

Motivation for the change: std::basic_string requires that
T implements std::char_traits and standard library provides specializations only
for the following types: char, char16_t, char32_t, wchar_t as per [1].

Note that this has been pointed out during a review previously [2], but made its
way back into the code in other places.

libc++19 has dropped implementations of std::char_traits for types not required
by the standard [3].

The base template for std::char_traits has been removed in LLVM 19.
If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t
or a custom character type for which you specialized std::char_traits, your code will stop working.

[1] N4713, 24.2.1 Character traits [char.traits] (C++17)
[2] https://www.github.com/diffblue/cbmc/pull/5277#discussion_r396609205
[3] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

---

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

---

Discovered when building cbmc for darwin with clang-19 NixOS/nixpkgs#371275 (comment).

This change should be basically an NFC related to portability, so no tests and documentation updates seem to be necessary.

[kroening]

Needs clang-format, but otherwise good!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.78%. Comparing base (5ae1452) to head (684bf42).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8559      +/-   ##
===========================================
- Coverage    78.82%   78.78%   -0.04%     
===========================================
  Files         1730     1730              
  Lines       199101   199189      +88     
  Branches     18317    18329      +12     
===========================================
- Hits        156939   156938       -1     
- Misses       42162    42251      +89     

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[xokdvium]

Thanks! Applied the clang-format diff, so the lint job should pass now.