SMT2 backend: reduction operators

[kroening]

This adds the reduction operators to the SMT2 backend.

• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.06%. Comparing base (1b41bbe) to head (6f61d03).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8964   +/-   ##
========================================
  Coverage    81.06%   81.06%           
========================================
  Files         1704     1704           
  Lines       210005   210084   +79     
  Branches        73       73           
========================================
+ Hits        170235   170315   +80     
+ Misses       39770    39769    -1     

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