rust: add logging utils This change adds the `log` crate, the community-approved logging facade backed by Rust core developers as well as the logging-consumer crate `simple_logger` to build a foundation for logging from Rust. Using this setup allows us to choose how to log depending on the way `hg-core` is used: if it's within the context of `hg-cpython`, we might not want to use it the same way as with a direct cli for example. Differential Revision: https://phab.mercurial-scm.org/D8252