We see SimpleActorTest timing out a lot in our CI when LAWS is used,
and from the logs it appears likely that this is a valid
timeout (versus the test just being stuck). Let us increase the test
timeout and see if it helps. Or if it is a "real" timeout.

A spurious wake-up can be produced by the new UnblockOnMainActorTest which
triggers the assert(!mpscQueue.empty()) in UnboundedBlockingMpscQueue::get.

Those spurious wake-ups are possible because the push and wake-up pair in
UnboundedBlockingMpscQueue::put are not atomic.
The following sequence diagram demonstrates a spurious wake-up:

   T1          T2            Q
   .           .            { }
  put(e)       .            { }
 push 54-57    .            {e}
   .         get()          {e}
   .        consume e       { }
   .           .            { }
   .         get()          { }
   .         block          { }
 unblock       .            { }
   .           .            { }
   .         wakeup         { }
   .           .            { }
               X
       assert(!queue.Empty())

To deal with spurious wake-ups we recheck the wake-up condition (a non empty queue)
and block again if we find it empty.
We assume spurious wake-ups are rare because it was difficult to reproduce them
even with a dedicated Test (the new UnblockOnMainActorTest) therefore we declare
the empty queue branch as unlikely.

Fixes #4.

We are unable to switch to C++20 until
https://github.com/mesonbuild/meson/issues/8084 is fixed.

In debug builds, the schedule() method is now guarded by an assert(),
as due to this change, the schedule() method will no longer work from
everywhere.

This also improves the worker sleep method.

This reverts commit b33ac93c.

Now that logging is done independently from NDEBUG, we can re-enable
"undef NDEBUG for test executables". The rationale that we want to
undefine NDEBUG for test executable is that we want, even in release
builds, asserts in tests enabled. Note that this also applies to
asserts in the emper core library, if those are in *headers*.

Disable the userpace-rcu support by default.
Our used userspace-rcu flavor memb is rather new and not available in
liburcu version 0.10 available in debian buster.
This change switches from using DCE and "if constexpr" to the C
preprocessor so the library is only needed when the userspace-rcu support
is actually enabled.

And fix the #include issues reported by the newer IWYU version.

The explicit -UNDEBUG was introduced in the CMake to meson migration (62900cd1)
but as far as I can tell it was not present in the CMake tests definition.

It makes tests less reliable because it enables only some debug messages but not
all.
For example if emper is build with NDEBUG a test using a Semaphore
will output debug messages from the Semaphore but not from the Runtime.

Introduce a new Actor test using BinaryPrivateSemaphores and an Actor.
Multiple fibers are created which create a BPS on the stack, submit it to the actor
and wait on the semaphore.
The Actor simply signals each semaphore it receives.

The SimpleURCUTest creates a lock-less hash table and inserts values from
separate fibers and verifies the correct insertion.

This allows to run indiviual tests using the meson runner, e.g.

meson --repeat 100 SimpleActorTest

This should not change much, but ensure that this method is also
invoked in the test (even though it is not explicitly tested).

Otherwise meson will use a random value.

This test fails for yet unknown reasons.

The test reliable segfaulted when a debug build as used, because it
constructed a counting semaphore within the C main function.

The CPS already gets incremented by the
SynchronizedFiber. Initializing the CPS with FIBER_COUNT means that
the CPS will never become ready.