build: Add `make clippy` option for Rust linting.
Note that clippy is quite loud/obnoxious, as its name would imply.
This target isn't meant to run on CI, or even necessarily exit
cleanly. It's merely a way for developers to check if they've made
any glaring mistakes, or done awkward things they might not have
otherwise realised. We *can* configure it to be quieter in the
future, but it's probably not worth rabbitholing on.
* CLOSES #22156: https://bugs.torproject.org/22156