Add floatbv_mod_exprt and floatbv_rem_exprt classes

[kroening]

This documents the existing use of the floatbv_mod and floatbv_rem expressions.

Fixes #8276.

• 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 78.91%. Comparing base (fd13d3a) to head (6c2e056).
Report is 10 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8532      +/-   ##
===========================================
- Coverage    78.93%   78.91%   -0.03%     
===========================================
  Files         1728     1728              
  Lines       198634   198698      +64     
  Branches     18309    18449     +140     
===========================================
+ Hits        156795   156796       +1     
- Misses       41839    41902      +63     

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[tautschnig]

These changes resolve #8276.