rust-cpython: leverage upstreamed py_capsule_fn!() macro

rust-cpython: bump cpython crates to 0.3 Unblocks py_capsule_fn!().

rust-cpython: turn inline comments into non-doc comments

rust-cpython: fix signature of make_dirstate_tuple() Fortunately, the layout of PyObject {} is compatible with a C pointer, but we shouldn't rely on that. This also fixes the handling of NULL pointer.

rust-cpython: mark capsule function as unsafe

rust-cpython: add wrapper around decapsule_make_dirstate_tuple() There are a couple of safety issues. First, the returned function pointer must be unsafe. Second, its return value must be a raw pointer (i.e. python27_sys::PyObject), not a cpython::PyObject. The wrapper function will address these issues.