Implement P3815 - scope_association

[Cra3z]

This PR will implement P3815 - scope_association, which represents a possible association between a sender and an async scope.

[ClausKlein]

Please do pip install pre-commit and use it with: pre-commit run --all

We have to modes: with and w/o CXX_MODULES!

-- Detecting CXX compile features - done
-- BEMAN_USE_STD_MODULE=OFF
-- BEMAN_USE_MODULES=ON
-- CMAKE_CXX_SCAN_FOR_MODULES=ON

[ClausKlein]

Please respect the existing ifdefs

[coveralls]

coverage: 93.991% (+0.2%) from 93.745%
when pulling 8fa8782 on Cra3z:main
into f3dcb8f on bemanproject:main.

[dietmarkuehl]

This looks nice! Thanks!

[Cra3z]

Hi @ClausKlein, I've addressed all the requested changes.
Could you please take another look when you have time? Thanks!

[ClausKlein]

Hi @ClausKlein, I've addressed all the requested changes. Could you please take another look when you have time? Thanks!

I locks good to me