Fix multiplication and division of complex numbers

[tautschnig]

Multiplication and division of complex numbers are not just pointwise applications of those operations.

Fixes: #8375

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• 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 20.98765% with 64 lines in your changes missing coverage. Please review.

Project coverage is 78.32%. Comparing base (629dbcd) to head (e449479).

Files	Patch %	Lines
src/solvers/flattening/boolbv_floatbv_op.cpp	0.00%	46 Missing ⚠️
src/goto-programs/remove_complex.cpp	48.57%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8376      +/-   ##
===========================================
- Coverage    78.35%   78.32%   -0.03%     
===========================================
  Files         1726     1726              
  Lines       188424   188498      +74     
  Branches     18248    18252       +4     
===========================================
+ Hits        147631   147650      +19     
- Misses       40793    40848      +55     

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