port the first exercise from the old curriculum

src/about_variables.rs

@@ -1,29 +1,38 @@
-fn guess_this() -> i32 {
-    let one = 5;
-    let two = 7;
-    let three = 3;
-    let result = (one + two) / three;
-    return result;
+// Welcome to Rustlings! If you're here, that means you've either successfully
+// downloaded Rustlings, or are looking at this on GitHub. Either way, let me
+// introduce you to one of the most basic elements of Rust:
+//
+// === VARIABLES ===
+//
+// Variables are essentially little containers that hold, well, something. Think
+// of them as a little cardboard box that you put stuff into. What can you put
+// into a virtual cardboard box in Rust? All kinds of stuff, it turns out!
+// Numbers, words, sequences, and much more. Let's start out simple, though.
+// Here's our first exercise:
+
+pub fn exercise_one() {
+    let x = 5;
+    verify!(0, x, "Number assignment");
+    //      ^  ^
+    //      |  |
+    // What's  The variable
+    //  in it  itself
 }
 
-fn simple() -> &'static str {
-    let hello = "Hello World!";
-    return hello;
-}
-
-mod tests {
-    use super::*;
-
-    pub fn test_simple() {
-        verify!("Hello World!", simple(), "Simple example");
-    }
-
-    pub fn test_complicated() {
-        verify!(1, guess_this(), "Complicated example");
-    }
-}
+// Did you get all that? The "let" word basically tells us that we now want to
+// define a variable, and what follows it, the "x" is the name of the variable.
+// Each variable has a name, like a label you put on your cardboard box so you
+// don't confuse it with another, similar looking one.
+// The whole "verify!" deal essentially means that Rustlings is checking if you
+// solved the exercise correctly. It compares the first argument with the
+// second, so in this case "0" with "x", where "x" is the _value_ of the variable
+// we called "x". When you write "x", you pull out the cardboard box labelled "x"
+// and take out what's inside of it.
+// Speaking of which, what *is* inside of our "x" cardboard box? I don't think it's
+// "0"... do you know? Replace the "0" with the value of the variable we defined.
+// After that, run "cargo run" in your command line, and see if you put in the
+// right answer.
 
 pub fn exec() {
-    tests::test_simple();
-    tests::test_complicated();
+    exercise_one();
 }