rust-pyo3-sharedref: renamed PySharedRefCell to PyShareable There were two problems with the naming: - the PyO3 version is not based on `RefCell`. Rather than calling this `SomethingRwLock` we decided it was best to consider this an implementation detail. We mentioned it in the doc earlier on, but that is merely just bringing some reassuring context to the reader, not intended to be a needed structural explanation. - the data is not shared: it is ready to be shared To keep the changeset readable, we only make the very minimal changes to the tests to make them pass, i.e., not renaming local variables (this will be done in a later move).