Polyfill cetl::optional, extend the testing suite

[pavel-kirienko]

• Add optional and related entities such as make_optional, nullopt, in_place, etc.
• Add type_traits.hpp.
• Fix usage of CETL_PROJECT_VERSION in the build recipes.
• Disable C++ extensions in CETLVaSt (they were mistakenly left enabled).
• Add basic typelist operations to CETLVaSt that are needed for exhaustive combinatorial testing.
• Add the CETL_NODISCARD attribute.
• Add a useful feature to the GitHub Actions workflow: run CI jobs for all pushes where the head commit contains a hashtag with the name of the job; e.g., adding #verification to the commit message will run the verification job.

This PR is feature- and testing-complete but lacks the docs, which I am going to add next. The code is ready to be reviewed meanwhile. You may notice that the coverage reporting is odd because there are a few places where templates are not marked as instantiated while they actually are, also some branches in the comparison operators appear to be mislabeled as partially-covered.

The commit history is non-informative and should not be looked at, as it will be squashed anyway.

My next step is to finish #51 (which is blocked on cetl::optional) and then perhaps switch to #86, because there are certain similarities between optional and variant that will make it easier for me to do this part now rather than later.

[pavel-kirienko]

Regarding the naming convention used see #85

[thirtytwobits]

Still reviewing but I'll flush these comments. Most important is is_nothrow_swapable is not in C++14.

[pavel-kirienko]

Heads up: I tweaked two Doxygen settings to eliminate unhelpful warnings about undocumented enum members (well-named entities may not require explicit docs) and to automatically distribute docs to all group members to reduce doc copypasta.

[lydia-at-amazon]

Perused the code. Looks good. Should definitely be enough to fulfill our immediate use case in Libcyphal. Approving

[thirtytwobits]

We're still missing an example app under cetlvast/suites/docs/examples

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
97.7% Coverage on New Code
3.8% Duplication on New Code

See analysis details on SonarCloud

[thirtytwobits]

Please add a task to create a cetlvast/suites/docs/examples/ example for optional in the future.