Also keep the context size at 64 KiB (there was a comment that
errorneously indicated that the context size is 4 MiB).

I do not see a reason to do so any more (there probably never was).

Use constexpr for memory manager in context manager

See merge request !330

[build] Check if std::atomic wait()/notify{_all}() is avaialble

See merge request !326

use emper_fibril in continuation-stealing tests

See merge request !328

Add support for continuation stealing

See merge request !327

Thanks to Nicolas Pfeiffer for writing the initial prototypical
implementation of continuation stealing and the cactus stack
mechanism, on which this is based.

Co-authored-by: Nicolas Pfeiffer <pfeiffer@cs.fau.de>

Fixes builds on Debian stable, which currently fail with

ccache c++ -Ieval/io_latency.p -Ieval -I../eval -Iemper -I../emper -Iemper/io -I../emper/io -Iemper/include -I../emper/include -I/usr/include -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -Wextra -Wpedantic -Werror -std=c++2a -O2 -g -DBOOST_ALL_NO_LIB -pthread -isystem../subprojects/liburing-liburing-2.0/src/include -isystemsubprojects/liburing-liburing-2.0/src/include -MD -MQ eval/io_latency.p/IoLatency.cpp.o -MF eval/io_latency.p/IoLatency.cpp.o.d -o eval/io_latency.p/IoLatency.cpp.o -c ../eval/IoLatency.cpp../eval/IoLatency.cpp: In function ‘void watchFunc()’:
../eval/IoLatency.cpp:45:17: error: ‘struct std::atomic<bool>’ has no member named ‘wait’
   45 |   readSubmitted.wait(false, std::memory_order_acquire);
      |                 ^~~~

Introduce AbstractFiber as superclass of all scheduled control flows

See merge request !325

It is safe to perform a relaxed load in isRunnable(), since the
information is potentially outdated immediately after it was looked
at.

Also, since Fiber::isRunnable() is a virtual method, keeping its
definition in the header has no real advantage, hence we move it out
of the header.

[io/Stats] don't try to print worker stats when using a single uring

See merge request !324

add io latency evaluation

See merge request !323

* increase iterations to 10K this produced more stable averages
* add options to dump all measurements
* add baseline benchmark using two threads and read/write syscalls
* add iouring benchmark using two threads and iouring to read

Test more Fiber functionality in Future callbacks: blocking and spawning.

It uses a watch thread writing to eventfd and a ReadFuture with a
Callback to measure the latency between writing to the eventfd and
executing the resulting IO Continuation.

The worker submitting the read request starts a busy loop after the
submission to enforce that the IO-continuation is delayed on this
worker.

[CI] Move iwyu and clang-tidy out of the smoke-test stage

See merge request !322

[CI] Bump container image to 1.20

See merge request !321

Various improvements

See merge request !320

[Runtime] Set a name for every worker thread

See merge request !319

[Context] Fix DBG message

See merge request !318

The thread local currentContext was already set to nullptr when we
logged it. Use the right field instead.

[meson] add option to ignore wakeup hints

See merge request !316

I think wakeup hints should never be ignored but having the option
seams usefull to observe their benefits/cost.

[Runtime] Move EMPER_* environment variable handling into module

See merge request !314

[ScheduleOnTest] block worker thread if IO is not available

See merge request !315

That debug statement is noisy, as it is emitted all the time,
irregardless whether or not the environment variable is set.

Also add a DBG message that is emitted if such a EMPER_* environment
variable has been processed.

The alpha fiber waits to ensure the worker threads are suspended before
starting to schedule work on specific threads.
emper::sleep uses an AlarmFuture and thus needs emper to be build with IO
support. If IO is not available we now just block the whole worker thread.