rust-cpython: prepare for writing tests that require libpython What I wanted is to disable the "cpython/extension-module<ver>" feature while building tests executable, but that seems not doable. Instead, this patch adds new features dedicated for tests. The make rule is extracted so that we can easily run cargo tests. Added a minimal test of ref-sharing as an example. More tests will follow.

rust-cpython: make inner functions and structs of ref_sharing private Most of these methods were public because they had to be accessible from macro-generated functions. Some "unsafe" can be removed since we can guarantee the data consistency across non-public operations.

rust-cpython: keep Python<'a> token in PyRefMut This just clarifies that the GIL is obtained while PyRefMut is dereferenced, so there's no need of extra acquire_gil() to drop the reference.

rust-cpython: require GIL to borrow immutable reference from PySharedRefCell Since the inner value may be leaked, we probably need GIL to guarantee that there's no data race. inner(py).borrow() is replaced with inner_shared(py).borrow(), which basically means any PySharedRefCell data should be accessed through PySharedRef wrapper.