Add CI workflow for GitHub Actions

[striezel]

It's based on the workflow file of bit-lite (https://github.com/martinmoene/bit-lite/blob/master/.github/workflows/ci.yml), so it probably should work without further changes.

As a side effect, this PR also contributes to martinmoene/nonstd-lite-project#70 by providing a workflow that already has the newer version of actions/checkout.

[striezel]

It did not work as expected (see https://github.com/martinmoene/optional-fun-lite/actions/runs/3595731162/jobs/6055579375 and others). It looks like the header file from optional-lite is required for compilation, too:

[ 33%] Building CXX object test/CMakeFiles/optional-fun-lite-cpp17.t.dir/optional-fun-main.t.cpp.o
<command-line>: fatal error: ../../optional-lite/include/nonstd/optional.hpp: No such file or directory
compilation terminated.

This means that the the CI workflows will probably need an additional step that pulls in the code from optional-lite.

[martinmoene]

@striezel

It has been a long time since I've looked at optional-fun-lite.
There are several tests, but I can't remember how mature it is (or not).
Setting up a Readme will help with discovering that too :)

Thank you very much for all the PRs!