This is a collection with unique-ownership,
and the ability to create all content objects in ''RAII style''
from the constructor call, using a ''population functor''.
It is often used to maintain a collection of resource handles,
and especially to start a series of threads (for testing).
With the variadic λ-captures from C++20, it is possible to rewrite
the most frequently used population functors to be easier to use.
Notably we can now bind constructor arguments, that will be passed
to each of the created elements — a prominent example being the
thread function when launching several threads. Furthermore,
I added a constructor variant that consumes a Lumiera Iterator.
Adding this kind of generic variants is possible now with C++20,
since overload resolution takes concepts into account.
This resolves a long-standing issue:
we could not use proper include paths for test-helper code that was defined alongside the tests.
Since some test-helper code must now be relocated into the »Vault«-Layer,
I took the opportunity to finally resolve this annoyance. The solution is two-fold
* introduce a dedicated test-support library
* remove the quirks that were necessary to support some plain-C tests
Together, this allows to raise the include-directory by one level,
and then to adjust all folder names so that they match the namespace
the buildsystem will now pick up and link
all test cases according to the layer, e.g.
backend tests will automatically be linked
against the backend + library solely.
