symbol_exprt C++-style API

[kroening]

This replaces the Java-style getter/setter in symbol_exprt by C++ style getter/setters.

• 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.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a 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 84.60452% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (844c8e6) to head (aed51dd).

Files with missing lines	Patch %	Lines
src/analyses/custom_bitvector_analysis.cpp	22.58%	24 Missing ⚠️
src/util/find_symbols.cpp	52.00%	12 Missing ⚠️
src/analyses/escape_analysis.cpp	57.69%	11 Missing ⚠️
src/analyses/global_may_alias.cpp	0.00%	5 Missing ⚠️
...bytecode_convert_method/convert_invoke_dynamic.cpp	0.00%	3 Missing ⚠️
src/ansi-c/goto-conversion/builtin_functions.cpp	66.66%	3 Missing ⚠️
src/goto-cc/linker_script_merge.cpp	40.00%	3 Missing ⚠️
src/goto-programs/interpreter.cpp	70.00%	3 Missing ⚠️
src/statement-list/statement_list_typecheck.cpp	90.00%	3 Missing ⚠️
src/analyses/interval_domain.cpp	71.42%	2 Missing ⚠️
... and 34 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8824      +/-   ##
===========================================
- Coverage    80.00%   80.00%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188332   188314      -18     
  Branches        73       73              
===========================================
- Hits        150679   150662      -17     
+ Misses       37653    37652       -1     

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