operator== for exprt and bool, int, nullptr_t

[tautschnig]

Introduces comparison operators for more concise code, and deprecates exprt::is_{true,false,zero,one} in favour of these.

Please review commit-by-commit.

• 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/
• 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 80.33827% with 93 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (724f2f7) to head (b89c75d).
⚠️ Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_expr.cpp	77.01%	20 Missing ⚠️
src/goto-instrument/goto_program2code.cpp	71.42%	6 Missing ⚠️
src/analyses/custom_bitvector_analysis.cpp	0.00%	5 Missing ⚠️
src/analyses/invariant_set.cpp	0.00%	5 Missing ⚠️
src/util/expr.cpp	0.00%	4 Missing ⚠️
src/goto-checker/counterexample_beautification.cpp	0.00%	3 Missing ⚠️
src/goto-instrument/dot.cpp	0.00%	3 Missing ⚠️
src/util/interval.cpp	62.50%	3 Missing ⚠️
src/util/simplify_expr.cpp	84.21%	3 Missing ⚠️
src/util/simplify_expr_int.cpp	85.00%	3 Missing ⚠️
... and 27 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8675      +/-   ##
===========================================
- Coverage    80.48%   80.47%   -0.02%     
===========================================
  Files         1698     1698              
  Lines       208477   208496      +19     
  Branches        73       73              
===========================================
- Hits        167789   167781       -8     
- Misses       40688    40715      +27     

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[feliperodri]

Changes in the contract module LGTM.