Bootstraping PyO3 Rust bindings

This experiments with the PyO3 Python bindings for Rust, which is now the official successor of rust-cpython.

This MR includes enough conversion utilities that it can reimplement the mercurial.rustext.dagop module. In particular, it solves the question to grab references to the inner core Rust index from the inner revlog exposed in the current rust-cpython bindings.

THe new bindings are actually plugged in, so that the test suite use them and performance assessment can be made. The rust-cpython code is not removed, so that back-and-forth comparisons are still easy to make.