SMT2: allow natural-typed shift distance

[kroening]

This adds conversion for shifts with natural-typed shift distance (in addition to integer-typed) 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 78.90%. Comparing base (162e0f7) to head (d325915).
Report is 27 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8519      +/-   ##
===========================================
+ Coverage    78.84%   78.90%   +0.06%     
===========================================
  Files         1728     1728              
  Lines       198775   198622     -153     
  Branches     18406    18394      -12     
===========================================
+ Hits        156719   156720       +1     
+ Misses       42056    41902     -154     

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