rust-cpython: start cpython crate bindings
This changeset introduces the hg-cpython crate,
that compiles as a shared library holding a whole
Python package (mercurial.rustext), with only the empty
'ancestor' submodule for now.
Such bindings will be easier and safer to develop and maintain
that those of `hg-direct-ffi`.
They don't involve C code, only unsafe Rust that's mostly isolated
within the cpython crate.
The long-term goal would be to import the provided modules, such
as rustext.ancestor with mercurial.policy.importmod, same as
we already do with cext modules.
Differential Revision: https://phab.mercurial-scm.org/D5434
copyright (c) 2006-2010 David JEAN LOUIS
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