Add basic combinators for generic and type-safe composition of nullables

tests: Port test cases to Catch2

And use its BDD-style that fits nicely with Catch2 support for names
that do not need to be valid C++ identifiers.

Given that absent does not use mocks, a simpler framework such as Catch2
might be a better option than GoogleTest right now.

Moreover, the tests are now simplified in order to reduce noise and
improve readability rather than other attributes (e.g. run-time
performance), for instance via passing by-value for lambdas instead of
const ref, etc.