rust-index: add a struct wrapping the C index Implementing the full index logic in one go is journey larger than we would like. To achieve a smoother transition, we start with a simple Rust wrapper that delegates allwork to the current C implementation. Once we will have a fully working index object in Rust, we can easily start using more and more Rust Code with it. The object in this patch is functional and tested. However, multiple of the currently existing rust (in the `hg-cpython` crate) requires a `Graph`. Right now we build this `Graph` (as cindex::Index) using the C index passed as a PyObject. They will have to be updated to be made compatible. Differential Revision: https://phab.mercurial-scm.org/D7655