Refactored parsing rules for function contracts

[aalok-thakkar]

The following four changes were made:

1. cprover_contract renamed to cprover_function_contract.
2. cprover_contract_sequence renamed to cprover_function_contract_sequence.
3. cprover_contract_sequence_opt renamed to cprover_function_contract_sequence_opt.
4. Refactored the assigns clause in cprover_function_contract by creating cprover_contract_assigns_opt.

This was done so we can reuse cprover_contract_assigns_opt for future use in Loop Contracts.

• 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.

[codecov]

Codecov Report

Merging #6195 (d242154) into develop (0002950) will increase coverage by 7.92%.
The diff coverage is n/a.

❗ Current head d242154 differs from pull request most recent head a2705f1. Consider uploading reports for the commit a2705f1 to get more accurate results

@@             Coverage Diff             @@
##           develop    #6195      +/-   ##
===========================================
+ Coverage    67.40%   75.32%   +7.92%     
===========================================
  Files         1157     1456     +299     
  Lines        95236   161128   +65892     
===========================================
+ Hits         64197   121376   +57179     
- Misses       31039    39752    +8713     

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/mathematical_expr.cpp	50.00% <0.00%> (-50.00%)	⬇️
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%)	⬇️
... and 1435 more

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[feliperodri]

Changes LGTM, but we should explain why they are necessary in the commit messages.