A Bad Singly-Linked Stack

This one's gonna be by far the longest, as we need to introduce basically all of Rust, and are gonna build up some things "the hard way" to better understand the language.

We'll put our first list in src/first.rs. We need to tell Rust that first.rs is something that our lib uses. All that requires is that we put this at the top of src/lib.rs (which Cargo made for us):

// in lib.rs
pub mod first;