use zero_extend_exprt in SMT2 front-end

[kroening]

This replaces the use of a typecast expression by zero_extend_exprt when parsing an SMT-LIB2 zero_extend expression.

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• 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).
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• 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 (fa299c5) to head (7687a69).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8493      +/-   ##
===========================================
+ Coverage    78.63%   78.93%   +0.30%     
===========================================
  Files         1728     1728              
  Lines       199408   198654     -754     
  Branches     18316    18352      +36     
===========================================
+ Hits        156797   156810      +13     
+ Misses       42611    41844     -767     

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[tautschnig]

While at it: please change to id == ID_zero_extend