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