rust-index: add a `inner` method to the Index struct We are about to introduce a Rust wrapper to the C index. It will be exposed as a Python object. As a start all call will be forwarded to an internal index object (as used now). For this forwarding to works, we need to access that index object (from the C extensions). We adds a method doing to on the existing `Index` rust struct. Differential Revision: https://phab.mercurial-scm.org/D7654