introduce goto_programt::instructiont::output

[kroening]

This introduces goto_programt::instructiont::output as a more idiomatic
substitute for goto_programt::output_instruction. The identifier
parameter is no longer in use, and is thus removed from the API.

• 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 #6894 (b4d69dd) into develop (000c152) will increase coverage by 0.00%.
The diff coverage is 92.43%.

@@           Coverage Diff            @@
##           develop    #6894   +/-   ##
========================================
  Coverage    77.80%   77.81%           
========================================
  Files         1567     1568    +1     
  Lines       179921   179980   +59     
========================================
+ Hits        139993   140055   +62     
+ Misses       39928    39925    -3     

Impacted Files	Coverage Δ
src/memory-analyzer/gdb_api.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
src/solvers/flattening/boolbv_quantifier.cpp	93.27% <50.00%> (+0.84%)	⬆️
src/goto-programs/goto_program.cpp	81.71% <75.00%> (+0.42%)	⬆️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.65% <96.72%> (-0.23%)	⬇️
src/ansi-c/expr2c.cpp	67.63% <100.00%> (ø)
src/goto-analyzer/unreachable_instructions.cpp	89.01% <100.00%> (-1.94%)	⬇️
src/goto-programs/goto_program.h	90.75% <100.00%> (+0.05%)	⬆️
src/goto-programs/show_goto_functions.cpp	76.31% <100.00%> (ø)
... and 13 more

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