Handle crashes via signal handlers. Switch python's revng.api from
a normal python function to faulthandler, which works also in the
case of harsher interruptions (e.g. SIGABRT).
rp_shutdown must be called after all the owning pointers given by
PipelineC are freed, in order to guarantee this easily, the python
ApiWrapper will use an atomic counter, which when shutdown has been
signaled and counter reaches zero will automatically call rp_shutdown
`revng.api` now autodetects files (libraries, pipeline yamls) for
initialization via the same mechanism used by `revng.cli`. This removes
the need to pass them via environment variables from `revng daemon`
and allows easy interaction with python's C API.
Since PipelineC is not thread-safe, add a lock to all function calls to
it to avoid any thread safety-related issues.
At the same time, use a thread pool to run "expensive" PipelineC
functions in GraphQL (specifically `produce_target` and `run_analysis`)
to avoid problems due to the cooperative nature of Python coroutines.
Giacomo Vercesi