introduce goto_instruction_codet

[kroening]

This introduces an alias for codet for use in goto_instructiont, to convey
the information that goto_instructiont expects a specific subset of the
codet types.

It is anticipated that goto_instruction_codet will eventually be a distinct
type.

• 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).
• 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 #6854 (0de72ad) into develop (a188777) will increase coverage by 0.00%.
The diff coverage is 97.82%.

@@           Coverage Diff            @@
##           develop    #6854   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files         1567     1567           
  Lines       179707   179713    +6     
========================================
+ Hits        139797   139803    +6     
  Misses       39910    39910           

Impacted Files	Coverage Δ
src/goto-programs/goto_program.h	90.27% <94.11%> (+0.02%)	⬆️
src/goto-programs/goto_instruction_code.h	100.00% <100.00%> (ø)

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