cmdlinet: add value_opt methods

[kroening]

This adds an alternative to cmdlinet::get_value, which returns std::optional, as opposed to defaulting to an empty string when the option isn't set.

• 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

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.85%. Comparing base (7eef276) to head (3c9d699).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/goto-cc/armcc_mode.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8525      +/-   ##
===========================================
+ Coverage    78.58%   78.85%   +0.26%     
===========================================
  Files         1728     1728              
  Lines       199522   198844     -678     
  Branches     18374    18379       +5     
===========================================
+ Hits        156792   156795       +3     
+ Misses       42730    42049     -681     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tautschnig]

I had the same thought when seeing your hw-cbmc PR...

[tautschnig]

How about changing this API to become value_or? So that it would be used like this: cmdline.value_or("log", "-")?

[kroening]

Might be confusing given that it is the same method name as std::optional::value_or. Right now what you do is only a little bit longer: cmdline.value_opt("log").value_or("-").

[kroening]

We could consider operator()? Then it would be cmdline("log").value_or("-")

[tautschnig]

Perhaps value_or isn't the best name to choose, but it still seems like a nuisance when the only thing we really want is value_opt immediately followed by value_or.

[kroening]

There is some (very modest) benefit in doing

auto foo_opt = cmdline.value_opt("foo");
if(foo_opt) ....  foo_opt.value()

The benefit is that you avoid the double lookup in the argument map that you'd do with cmdline.isset("foo") followed by cmdline.get_value("foo"). But of course not measurable.