* Add feature controlling the global reference pool to enable avoiding its overhead.
* Document reference-pool feature in the performance guide.
* Invert semantics of feature to disable reference pool so the new behaviour becomes opt-in
* Remove delayed reference count increments as we cannot prevent reference count errors as long as these are available
* Adjust tests to be compatible with disable-reference-pool feature
* Adjust tests to be compatible with py-clone feature
* Adjust the GIL benchmark to the updated reference pool semantics.
* Further extend and clarify the documentation of the py-clone and disable-reference-pool features
* Replace disable-reference-pool feature by pyo3_disable_reference_pool conditional compilation flag
Such a flag is harder to use and thereby also harder to abuse. This seems
appropriate as this is purely a performance-oriented change which show only be
enabled by leaf crates and brings with it additional highly implicit sources of
process aborts.
* Add pyo3_leak_on_drop_without_reference_pool to turn aborts into leaks when the global reference pool is disabled and the GIL is not held
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