SMT2 back-end: flatten with_exprt operands

[tautschnig]

Solvers without datatypes support require flattening towards bitvectors. This is also true for array-typed expressions when the array is not at the top level (despite possible array-theory support). This changes makes sure we will flatten such array-typed expressions.

• 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

Attention: Patch coverage is 53.57143% with 13 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (e88ed5f) to head (1bb5b81).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	53.57%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8670      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207387   207394       +7     
  Branches        73       73              
===========================================
+ Hits        166728   166733       +5     
- Misses       40659    40661       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[tautschnig]

Note to reviewers: #8637 includes a test set-up with tests passing once the changes in this PR are applied. I am working on extracting a minified variant thereof to include as regression test.