Move is_{true,false,zero,one} from exprt to constant_exprt

[tautschnig]

true/false/zero/one are constants, so only for a constant_exprt those methods can ever evaluate to true.

This commit both deprecates the exprt methods and removes all their uses from the code base.

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• 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).
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[codecov]

Codecov Report

Attention: Patch coverage is 79.36085% with 155 lines in your changes missing coverage. Please review.

Project coverage is 80.38%. Comparing base (7a6e2f7) to head (1a6ab22).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/std_expr.cpp	71.92%	16 Missing ⚠️
src/goto-instrument/goto_program2code.cpp	70.00%	12 Missing ⚠️
src/analyses/invariant_set.cpp	0.00%	10 Missing ⚠️
src/analyses/custom_bitvector_analysis.cpp	0.00%	9 Missing ⚠️
src/goto-instrument/dot.cpp	0.00%	9 Missing ⚠️
src/goto-checker/counterexample_beautification.cpp	0.00%	8 Missing ⚠️
src/analyses/goto_rw.cpp	12.50%	7 Missing ⚠️
src/util/simplify_expr_int.cpp	81.08%	7 Missing ⚠️
...goto-instrument/accelerate/sat_path_enumerator.cpp	0.00%	6 Missing ⚠️
src/util/interval.cpp	60.00%	6 Missing ⚠️
... and 29 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8664      +/-   ##
===========================================
- Coverage    80.39%   80.38%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207134   207448     +314     
  Branches        73       73              
===========================================
+ Hits        166520   166759     +239     
- Misses       40614    40689      +75     

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[feliperodri]

All changes in the contract-related modules LGTM.

[kroening]

This does increase verbosity.

[kroening]

I think I'd prefer that we make the constructors of true_exprt, false_exprt etc. return a lazily constructed static object; we could then write expr == true_exprt{} and the like.

[kroening]

For numbers, this will be a bit more verbose, but I don't have a great idea.

[tautschnig]

Can I suggest to take three different approaches:

1. is_true and is_false get replaced by == true_exprt{} and == false_exprt{}, respectively per your suggestion.
2. We get rid of is_one: it isn't used widely and we actually have multiple implementations with the same name already.
3. We do for is_zero what this PR proposes.

Would that work?