rust-cpython: implement Graph using C parents function

Authored by gracinet.

Description

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