Refactoring code contracts implementation

[feliperodri]

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[SaswatPadhi]

Overall LGTM, some minor comments below:

[codecov]

Codecov Report

Merging #6234 (5d0ffe0) into develop (0002950) will increase coverage by 8.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #6234      +/-   ##
===========================================
+ Coverage    67.40%   75.48%   +8.07%     
===========================================
  Files         1157     1463     +306     
  Lines        95236   161245   +66009     
===========================================
+ Hits         64197   121719   +57522     
- Misses       31039    39526    +8487     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/util/string_container.cpp	52.94% <0.00%> (-47.06%)	⬇️
src/solvers/prop/prop.cpp	42.85% <0.00%> (-41.76%)	⬇️
src/solvers/flattening/boolbv_member.cpp	53.65% <0.00%> (-38.65%)	⬇️
src/cpp/cpp_storage_spec.cpp	65.00% <0.00%> (-35.00%)	⬇️
src/util/cmdline.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/array_pool.h	66.66% <0.00%> (-33.34%)	⬇️
src/solvers/strings/string_refinement.h	66.66% <0.00%> (-33.34%)	⬇️
...rs/strings/string_concatenation_builtin_function.h	0.00% <0.00%> (-33.34%)	⬇️
src/cbmc/c_test_input_generator.cpp	60.00% <0.00%> (-30.33%)	⬇️
src/analyses/local_cfg.h	75.00% <0.00%> (-25.00%)	⬇️
... and 1443 more

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[SaswatPadhi]

Thanks for the refactoring! LGTM

[martin-cs]

This kind of refactor is often hard to review so at best I think this is OK and I trust @feliperodri 's judgement here. There is a 40% reduction in code or so and it doesn't break the tests so that seems like a good start.