.arcconfig

rust-cpython: allow mutation unless leaked reference is borrowed

In other words, mutation is allowed while a Python iterator holding PyLeaked
exists. The iterator will be invalidated instead.

We still need a borrow_count to prevent mutation while leaked data is
dereferenced in Rust world, but most leak_count business is superseded by
the generation counter.

decrease_leak_count(py, true) will be removed soon.

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