embed asyncio_gevent within BLISS code, split greenlet_utils module in submodules

probably closes #3552 (closed)

asyncio_gevent was a separate project from Frederique Mittelstaedt (see https://github.com/gfmio/asyncio-gevent) but the project is unmaintained nowadays and we are only interested in the 'asyncio on top of gevent' scenario, so it has been decided to take the code we need, as the permissive license allows, and to not rely on an external dependency for this. We can further improve/adapt asyncio_gevent to BLISS if needed, as we did to fix the following issue from github: https://github.com/gfmio/asyncio-gevent/issues/8