allow multiple assigns clauses

[kroening]

This changes the parser to allow multiple assigns clauses in a contract.
The meaning of multiple clauses is the same as the meaning of one clause
that contains the concatenation of the target lists of the given clauses.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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).
• 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.

[SaswatPadhi]

Thanks! LGTM

[codecov]

Codecov Report

Merging #6226 (af22e6c) into develop (0002950) will increase coverage by 8.05%.
The diff coverage is n/a.

❗ Current head af22e6c differs from pull request most recent head ef74099. Consider uploading reports for the commit ef74099 to get more accurate results

@@             Coverage Diff             @@
##           develop    #6226      +/-   ##
===========================================
+ Coverage    67.40%   75.46%   +8.05%     
===========================================
  Files         1157     1459     +302     
  Lines        95236   161447   +66211     
===========================================
+ Hits         64197   121841   +57644     
- Misses       31039    39606    +8567     

Flag	Coverage Δ
cproversmt2	?
regression	?
unit	?

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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 1438 more

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