Add array exercise from @ruipserra!!! 🎆🎆🎆

Connects to #29.

primitive_types/primitive_types3.md

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+// Create an array with at least 100 elements in it where the ??? is. 
+// Scroll down for hints!
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+fn main() {
+    let a = ???
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+    if a.len() >= 100 {
+        println!("Wow, that's a big array!");
+    } else {
+        println!("Meh, I eat arrays like that for breakfast.");
+    }
+}
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+// There's a shorthand to initialize Arrays with a certain size that does not 
+// require you to type in 100 items (but you certainly can if you want!)
+// Check out the Primitive Types -> Arrays section of the book:
+// http://doc.rust-lang.org/stable/book/primitive-types.html#arrays
+// Bonus: what are some other things you could have that would return true
+// for `a.len() >= 100`?