Supporting Destructors in the TSS API

We have been looking in PyO3 at using the Python Thread-Specific-Storage API to provide thread-local storage for extensions which don’t want to depend on the implementation in the Rust standard library. One feature which seems to be missing to me is the ability to register a destructor when the thread exits; it seems to me that there’s no option but to leak allocated thread-local values when their owning thread exits. To support this pthread_key_create accepts a function-pointer second argumen...