Add invariant that multiplication uses operands of the same size

[tautschnig]

Our implementations swap operands, and determine the size of the result from the size of the operand(s). Make sure we produce consistent-width results.

• 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.

[kroening]

That precondition is already in bv_utilst::unsigned_multiplier_no_overflow; I suppose we could add it to the other entry points to get an early error.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.67%. Comparing base (a8861ed) to head (f0b0aa2).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8241      +/-   ##
===========================================
- Coverage    79.67%   79.67%   -0.01%     
===========================================
  Files         1682     1682              
  Lines       195490   195491       +1     
===========================================
- Hits        155765   155750      -15     
- Misses       39725    39741      +16     

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