rust-cpython: make PyLeakedRef operations relatively safe This patch encapsulates the access to the leaked reference to make most leaked-ref operations safe. The only exception is leaked_ref.map(). I couldn't figure out how to allow arbitrary map operation safely over an unsafe static reference. See the docstring and inline comment for details. Now leak_immutable() can be safely implemented as the PyLeakedRef owns its inner data.

rust-cpython: put leaked reference in PyLeakedRef The next patch will make PyLeakedRef manage the lifetime of the underlying object. leak_handle.data.take() will be removed soon.