add multi-ary constructor for mult_exprt

[kroening]

This adds a constructor to mult_exprt that takes a vector of factors.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.93%. Comparing base (e8d3409) to head (7d5bfd9).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8510      +/-   ##
===========================================
+ Coverage    78.51%   78.93%   +0.41%     
===========================================
  Files         1728     1728              
  Lines       199553   198513    -1040     
  Branches     18303    18364      +61     
===========================================
  Hits        156687   156687              
+ Misses       42866    41826    -1040     

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