Add smt response storage classes

[thomasspriggs]

This PR adds smt response storage classes for type safe storage of parsed responses from SMT2 solvers.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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

Merging #6425 (e5db980) into develop (f6fa03c) will increase coverage by 0.02%.
The diff coverage is 99.33%.

@@             Coverage Diff             @@
##           develop    #6425      +/-   ##
===========================================
+ Coverage    75.98%   76.00%   +0.02%     
===========================================
  Files         1524     1527       +3     
  Lines       164299   164454     +155     
===========================================
+ Hits        124843   125000     +157     
+ Misses       39456    39454       -2     

Impacted Files	Coverage Δ
src/solvers/smt2_incremental/smt_responses.def	87.50% <87.50%> (ø)
src/solvers/smt2_incremental/smt_responses.cpp	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/smt_responses.h	100.00% <100.00%> (+100.00%)	⬆️
unit/solvers/smt2_incremental/smt_responses.cpp	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/smt_commands.h	100.00% <0.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <0.00%> (ø)
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	93.96% <0.00%> (+1.72%)	⬆️

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