SMT2 parser: add abbreviated versions of the rounding modes

[kroening]

This adds RNE, RNA, RTP, RTN, RTZ as abbreviated versions of the rounding modes.

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• 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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[codecov]

Codecov Report

Attention: Patch coverage is 39.13043% with 14 lines in your changes missing coverage. Please review.

Project coverage is 78.53%. Comparing base (d5cf498) to head (03ac764).
Report is 12 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_parser.cpp	39.13%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8539      +/-   ##
===========================================
- Coverage    78.58%   78.53%   -0.05%     
===========================================
  Files         1729     1729              
  Lines       199635   199772     +137     
  Branches     18311    18311              
===========================================
+ Hits        156887   156896       +9     
- Misses       42748    42876     +128     

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