This commit introduces simple unit test cases for the S/R (Sender
and Receiver) versions of the parallel algorithms where they did not
exist yet. Note that this is only the first part of these additional
test cases, they have not been implemented for every algorithm yet.

All usages of the tt::sync_wait sender consumer in the recently
added unit tests were replaced with equivalent function calls
in order to make the tests compatible with NVIDIA's stdexec.

This commit adds the remaining sender adaptor test cases. As of now,
it is not planned to add further test cases, as test cases were
added to all the algorithms where they are currently of interest.

This commit introduces a first batch of fixes for the
S/R versions of some parallel algorithms. However,
they are currently all build on a temporary solution
for an issue in the "then sender" and therefore not
final yet.

- Fly-by: comment out sender tests for algorithms that are not fixed yet

…ntation

Co-authored-by: isidorostsa <[email protected]>

The guard prevents the use of invocables returning bool in the
bulk_async_execute method of the explicit_scheduler_executor to
avoid data races. Using such invocables can cause issues because
std::vector<return type of the invocable> may be simultaneously
written to by multiple threads, which is problematic
with std::vector<bool> due to its space optimizations.

This commit introduces a new set of fixes for the
S/R versions of several more parallel algorithms and
reworks of some previous, buggy fixes. Additionally,
new unit test cases have been added for some of the
algorithms to cover edge cases such as empty ranges in
case they could become pitfalls for the S/R adaptations.
This led to the discovery of some general bugs in the
algorithms, which were fixed as flybys:
* `first_first_of` (+ regression test case)
* `ends_with`

Fly-by: Rebalance the two target lists

Flybys: add missing newline at EOF, fix typo

`stellargroup/build_env:14` uses Clang 10, which lacks support for
`std::identity`, a type explicitly required for enabling stdexec in
HPX. Since `stellargroup/build_env:16` uses Clang 12, switching
to it should resolve this issue.

This experimental fix uses an updated version of the build environment to
address the issues arising with Clnag-Format 12. Additionally, as flybys,
the compiler and the tools were upgraded to their most recent versions.
This build environment might therefore not be suitable for getting merged
and may be reverted in future commits.

This temporary commit disables that clang-tidy treats warnings as errors.
The reason for this is that the upgraded clang-tidy reports many new
warnings in various parts of HPX, causing the compilation to fail,
and addressing this is probably out of this PR's scope.

This commit temporarily disables clang-tidy and clang-format
in CircleCI to make the CI usable for this PR. These changes
will therefore be reverted later. Additionally, the spelling
mistakes were fixed and the Python dependencies in the build
environment adapted, so that building the documentation should
work again.

stdexec's implementation of sync_wait blocks the entire OS thread,
in accordance with P2300 [exec.sync.wait]. Without work stealing,
this causes deadlocks at the PX-thread level, leading to timeouts
in many of the sender-related tests. In the long term, this should
probably be fixed by implementing a custom sync_wait for HPX that
is compatible with stdexec's senders.

Flyby: fix bugs in `includes` and `search` algorithms

Flyby: Fix bug in `mismatch` algorithm

This macro was introduced for usage in some of the algorithm
fixes, but is actually unnecessary as one can simply
omit the parameter name.

Fly-by: Reenable S/R unit test for rotate

Remove the S/R unit tests of the algorithms that are not fixed yet,
since they will fail anyways and currently only pollute the test
folder. To resume work on these tests in the future, simply revert
this commit.

The unit tests for the S/R versions of the parallel algorithms are only configured
if HPX_WITH_STDEXEC is enabled. Additionally, the tests cases that are in headers
shared with the non-S/R tests are surrounded with macro guards checking if stdexec
is available.

* Add missing headers
* Update copyright comments
* Make formatting and naming more consistent

This reverts commit 83a180f.

This reverts the following commits:
c4f5276
6880e02
30d71fb
c41aac7
a60d958
22e5bc3
03cc5aa

Flyby: Fix formatting issues

This commits reverts other changes that are not related to the aims of
this PR, but which were not undone by the previous git reverts.

`transfer_just` was removed in the 10th revision of P2300, this
commit replaces its usage in the `explicit_scheduler_executor`.