SMT2: implement flattening for arrays

[kroening]

This adds flattening for arrays into the SMT2 backend, used, for instance, when lowering unions.

• 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.
• 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

Base: 78.03% // Head: 78.05% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (7fa94ab) compared to base (21e3260).
Patch coverage: 96.42% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7290      +/-   ##
===========================================
+ Coverage    78.03%   78.05%   +0.01%     
===========================================
  Files         1624     1624              
  Lines       187405   187422      +17     
===========================================
+ Hits        146249   146289      +40     
+ Misses       41156    41133      -23     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv_width.cpp	73.80% <0.00%> (ø)
src/ansi-c/expr2c.cpp	67.35% <100.00%> (+0.03%)	⬆️
src/goto-programs/goto_clean_expr.cpp	99.02% <100.00%> (ø)
src/goto-programs/goto_convert_side_effect.cpp	94.50% <100.00%> (+0.02%)	⬆️
src/goto-symex/goto_symex.cpp	98.64% <100.00%> (ø)
src/solvers/smt2/smt2_conv.cpp	66.36% <100.00%> (+0.88%)	⬆️

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