This commit adds a new crew field called `parent_flow`, evaluated when
the `Crew` instance is instantiated. The stacktrace is traversed to look
up if the caller is an instance of `Flow`, and if so, it fills in the
field.
Other alternatives were considered, such as a global context or even a
new field to be manually filled, however, this is the most **magical**
solution that was thread-safe and did not require public API changes.
70cd98035f