This patch includes content from the following places.
- Lina's "Getting Rusty: How to ship an XPCOM component in Firefox" slide deck.
- Zibi's "Rust <--> C/C++ FFI for newbies" gist.
It also links to Emilio's "FFI patterns #1 - Complex Rust data structures
exposed seamlessly to C++" blog post. I was going to include that content, but
it's very long, so I have omitted it for now.
Differential Revision: https://phabricator.services.mozilla.com/D81963
Some of the testing info is from the Oxidation wiki, and the logging info is
largely from a dev-platform email by Valentin. The other parts I wrote from
scratch.
The commit also makes some small improvements to the Rust build docs.
Differential Revision: https://phabricator.services.mozilla.com/D81017
