Goto instruction guard/condition API

[kroening]

• 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).
• 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 #6392 (d2801ee) into develop (af94148) will increase coverage by 0.00%.
The diff coverage is 83.01%.

@@           Coverage Diff            @@
##           develop    #6392   +/-   ##
========================================
  Coverage    75.97%   75.97%           
========================================
  Files         1523     1523           
  Lines       164191   164197    +6     
========================================
+ Hits        124748   124755    +7     
+ Misses       39443    39442    -1     

Impacted Files	Coverage Δ
src/goto-instrument/concurrency.cpp	0.00% <0.00%> (ø)
src/goto-instrument/dot.cpp	0.00% <0.00%> (ø)
src/goto-symex/symex_main.cpp	86.00% <60.00%> (+0.03%)	⬆️
src/goto-programs/goto_program.h	90.06% <66.66%> (-0.22%)	⬇️
src/goto-analyzer/taint_analysis.cpp	78.21% <75.00%> (-0.22%)	⬇️
src/goto-instrument/goto_program2code.cpp	69.33% <87.50%> (+0.12%)	⬆️
...sitivity/variable_sensitivity_dependence_graph.cpp	84.40% <100.00%> (ø)
src/goto-checker/symex_bmc.cpp	96.80% <100.00%> (ø)
src/goto-instrument/cover_instrument_branch.cpp	96.55% <100.00%> (+0.12%)	⬆️
src/goto-programs/goto_convert.cpp	91.76% <100.00%> (ø)
... and 5 more

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[martin-cs]

Seems reasonable.