goto_programt::output_instruction now uses format(...)

[kroening]

Goto programs are intended as an internal representation that is independent
of the specific input programming language. This commit switches the debug
output for the instructions of goto programs to format(...), which is also
designed to be language independent.

• 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/
• n/a 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 #6251 (75a16c7) into develop (5dd2519) will decrease coverage by 0.02%.
The diff coverage is 89.58%.

@@             Coverage Diff             @@
##           develop    #6251      +/-   ##
===========================================
- Coverage    76.22%   76.19%   -0.03%     
===========================================
  Files         1482     1482              
  Lines       161866   161886      +20     
===========================================
- Hits        123383   123350      -33     
- Misses       38483    38536      +53     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv.h	76.19% <ø> (ø)
src/ansi-c/parser.y	79.26% <87.50%> (ø)
src/goto-instrument/contracts/contracts.cpp	91.93% <88.67%> (ø)
src/goto-programs/goto_program.cpp	68.39% <90.32%> (+1.43%)	⬆️
src/ansi-c/c_typecheck_code.cpp	78.43% <100.00%> (ø)
src/ansi-c/expr2c.cpp	64.95% <0.00%> (-4.23%)	⬇️
jbmc/src/java_bytecode/expr2java.cpp	86.19% <0.00%> (-0.42%)	⬇️
src/util/format_expr.cpp	89.16% <0.00%> (+6.85%)	⬆️
... and 1 more

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[peterschrammel]

Is this the only place where we print codet? It might be worth moving the formatting into a util/format_code.h (not in this PR) otherwise.

[kroening]

The idea is to get rid of the use of codet in goto functions altogether; function calls are in fact the only instruction that doesn't have an alternative API yet.