pretty-print expression instructions

[kroening]

This adds pretty-printing for goto-program expression instructions.

• 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).
• 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 #6991 (09e1041) into develop (8eef445) will decrease coverage by 0.03%.
The diff coverage is 84.57%.

@@             Coverage Diff             @@
##           develop    #6991      +/-   ##
===========================================
- Coverage    77.84%   77.81%   -0.04%     
===========================================
  Files         1570     1570              
  Lines       180439   180682     +243     
===========================================
+ Hits        140468   140601     +133     
- Misses       39971    40081     +110     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-instrument/horn_encoding.cpp	72.70% <0.00%> (+0.43%)	⬆️
src/goto-symex/goto_symex_state.cpp	91.71% <ø> (ø)
src/goto-symex/goto_symex_state.h	100.00% <ø> (ø)
src/util/replace_symbol.cpp	78.20% <36.95%> (-10.10%)	⬇️
src/linking/linking.cpp	58.71% <50.00%> (-0.03%)	⬇️
src/util/rename_symbol.cpp	76.82% <69.38%> (-3.00%)	⬇️
src/ansi-c/c_typecheck_base.cpp	82.50% <92.30%> (+1.34%)	⬆️
src/goto-symex/field_sensitivity.cpp	91.27% <96.00%> (-0.24%)	⬇️
src/goto-instrument/contracts/contracts.cpp	94.31% <98.87%> (+0.21%)	⬆️
... and 23 more

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