Get a Rust -> C++ FFI and C++ -> Rust FFI example working

[vkottler]

We'll need to expose some configuration options for arguments to pass rustc. Overall still quite a bit to do for this, and more research to do.

From the horse's mouth: https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#using-extern-functions-to-call-external-code

See also: #48.

[vkottler]

Besides a technical example of one-off source files showing language interoperability, the larger question is how to get project and crate interoperability.

e.g. could a yambs project also exist as a Rust crate all in one?

[vkottler]

We should see what this is all about, too: https://conan.io/

Maybe we could integrate with it?

[vkottler]

We can put this aside until we have a legitimate need for FFI or package management outside of the GitHub + local source system we have now.