Avoid short-lived heap-allocated objects

[tautschnig]

valgrind --tool=dhat reports these as heap allocations that only have a
lifetime of a few hundred instructions, or even less than 100.

• 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 #6055 (f287288) into develop (ff5902e) will increase coverage by 0.03%.
The diff coverage is 91.66%.

@@             Coverage Diff             @@
##           develop    #6055      +/-   ##
===========================================
+ Coverage    74.25%   74.29%   +0.03%     
===========================================
  Files         1444     1444              
  Lines       157460   157444      -16     
===========================================
+ Hits        116928   116969      +41     
+ Misses       40532    40475      -57     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.h	94.11% <ø> (ø)
src/solvers/flattening/boolbv_case.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/boolbv_cond.cpp	0.00% <0.00%> (ø)
src/solvers/prop/prop.cpp	42.85% <0.00%> (-42.86%)	⬇️
src/solvers/prop/prop.h	84.37% <ø> (ø)
src/solvers/sat/cnf.h	83.33% <ø> (ø)
src/util/ssa_expr.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv.cpp	79.81% <50.00%> (+0.29%)	⬆️
src/goto-instrument/code_contracts.cpp	84.75% <100.00%> (+0.46%)	⬆️
src/goto-symex/field_sensitivity.cpp	90.90% <100.00%> (-0.06%)	⬇️
... and 51 more

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