Add SMT quantifier data structures

[thomasspriggs]

This PR adds the forall and exists SMT quantifier data structures, along with their conversion to string and unit tests.

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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 #7010 (4a0b099) into develop (4d5d529) will increase coverage by 0.01%.
The diff coverage is 96.57%.

@@             Coverage Diff             @@
##           develop    #7010      +/-   ##
===========================================
+ Coverage    77.85%   77.87%   +0.01%     
===========================================
  Files         1569     1569              
  Lines       180656   180817     +161     
===========================================
+ Hits        140648   140805     +157     
- Misses       40008    40012       +4     

Impacted Files	Coverage Δ
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <ø> (ø)
unit/solvers/smt2_incremental/smt_terms.cpp	93.23% <92.15%> (-0.67%)	⬇️
...rc/solvers/smt2_incremental/smt_to_smt2_string.cpp	95.65% <95.65%> (+1.24%)	⬆️
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/smt_terms.cpp	97.05% <100.00%> (+1.34%)	⬆️
src/solvers/smt2_incremental/smt_terms.def	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/smt_terms.h	100.00% <100.00%> (ø)
...it/solvers/smt2_incremental/smt_to_smt2_string.cpp	100.00% <100.00%> (ø)
src/goto-cc/gcc_cmdline.cpp	79.23% <0.00%> (ø)
src/goto-cc/armcc_cmdline.cpp	22.22% <0.00%> (ø)
... and 3 more

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[esteffin]

Looks good to me