SMT2 backend: generate simple symbols

[kroening]

• 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).
• 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

Merging #6940 (baba52b) into develop (40bd2b5) will increase coverage by 0.00%.
The diff coverage is 96.87%.

@@           Coverage Diff            @@
##           develop    #6940   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files         1568     1568           
  Lines       180329   180347   +18     
========================================
+ Hits        140290   140309   +19     
+ Misses       40039    40038    -1     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_tokenizer.h	96.96% <ø> (ø)
src/solvers/smt2/smt2_dec.cpp	73.27% <90.00%> (+2.10%)	⬆️
src/solvers/smt2/smt2_conv.cpp	67.90% <97.29%> (+0.13%)	⬆️
src/solvers/smt2/smt2_tokenizer.cpp	97.70% <100.00%> (ø)
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	94.30% <100.00%> (ø)
unit/solvers/smt2/smt2_conv.cpp	100.00% <100.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <100.00%> (ø)

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