add nand_exprt, nor_exprt, xnor_exprt, bitnand_exprt, bitnor_exprt

[kroening]

This adds classes for the nand, nor, bitnand, and bitnor expressions, documenting already existing expressions.

The comments clarify the meaning when those expressions are not binary.

• 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 (fd13d3a) to head (740f399).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8531   +/-   ##
========================================
  Coverage    78.93%   78.93%           
========================================
  Files         1728     1728           
  Lines       198634   198634           
  Branches     18309    18354   +45     
========================================
  Hits        156795   156795           
  Misses       41839    41839           

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