SMT2: implement check-sat-assuming

[kroening]

This adds support for SMT-LIB2 check-sat-assuming.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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).
• n/a 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 #6037 (211b539) into develop (4b819c2) will increase coverage by 0.10%.
The diff coverage is 64.70%.

@@             Coverage Diff             @@
##           develop    #6037      +/-   ##
===========================================
+ Coverage    74.02%   74.13%   +0.10%     
===========================================
  Files         1444     1444              
  Lines       157401   157442      +41     
===========================================
+ Hits        116521   116720     +199     
+ Misses       40880    40722     -158     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.cpp	60.36% <63.63%> (+0.29%)	⬆️
src/solvers/smt2/smt2_solver.cpp	83.11% <65.51%> (-2.25%)	⬇️
src/util/expr.cpp	75.73% <0.00%> (+1.47%)	⬆️
src/util/unicode.cpp	82.44% <0.00%> (+1.52%)	⬆️
src/util/json_irep.cpp	94.64% <0.00%> (+1.78%)	⬆️
src/linking/static_lifetime_init.cpp	98.63% <0.00%> (+2.73%)	⬆️
src/goto-checker/solver_factory.cpp	75.72% <0.00%> (+2.91%)	⬆️
src/util/std_types.cpp	88.88% <0.00%> (+4.93%)	⬆️
src/util/mp_arith.cpp	50.64% <0.00%> (+9.09%)	⬆️
... and 2 more

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