Update FFI example
Update FFI example

- Use meaningful names
- Clarify comments
- Fix C function declaration

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust