rust-cpython: implement Graph using C parents function We introduce the `Index` struct that wraps the C index. It is not intrinsically protected by the GIL (see the lengthy discussion in its docstring). Improving on this seems prematurate at this point. A pointer to the parents function is stored on the parsers C extension module as a capsule object. This is the recommended way to export a C API for consumption from other extensions. See also: https://docs.python.org/2.7/c-api/capsule.html In our case, we use it in cindex.rs, retrieving function pointer from the capsule and storing it within the CIndex struct, alongside with a pointer to the index. From there, the implementation is very close to the one from hg-direct-ffi. The naming convention for the capsule is inspired from the one in datetime: >>> import datetime >>> datetime.datetime_CAPI <capsule object "datetime.datetime_CAPI" at 0x7fb51201ecf0> although in datetime's case, the capsule points to a struct holding several type objects and methods. Differential Revision: https://phab.mercurial-scm.org/D5438