Update/rewrite "Calling Rust code from C" section

This https://doc.rust-lang.org/nomicon/ffi.html#calling-rust-code-from-c
( and even in the book here: https://doc.rust-lang.org/stable/book/ch19-01-unsafe-rust.html#calling-rust-functions-from-other-languages )

has no mention of all the other info needed to accomplish this, such as: compile as a cdylib by adding crate-type = [ "cdylib" ] in Cargo.toml [lib] section, or using cbindgen to generate a header file, or the gcc command line to compile the .cpp file.

(NOTE: the below link is now invalid as the author deleted their account.)
I've attempted all the needed stuff in a working example here.

[Gankra]

yes this chapter was transplanted from TRPL while I was away, and needs to be completely rewritten.