write smaller "FFI intro task"

[Lotterleben]

feedback to the main FFI task has been that it's a lot at once, so it'd be great to add a small warmup task that wraps a well-known C function like printf or so

[jamesmunns]

Assigning this to myself, as I will clean up and merge our smaller/more contained "Calculator FFI Around The World" exercise.

[jamesmunns]

Work in progress tracked here: https://github.com/ferrous-systems/calc-ffi-around-the-world