C library: implement {v,}snprintf

[tautschnig]

The model will non-deterministically alter the target string up to the bound given as argument to {v,}snprintf.

• 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 79.64%. Comparing base (1fd7011) to head (cd7459e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8239   +/-   ##
========================================
  Coverage    79.64%   79.64%           
========================================
  Files         1684     1684           
  Lines       195606   195606           
========================================
  Hits        155791   155791           
  Misses       39815    39815           

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