Add new section on trait bounds.

exercises/04_traits/08_sized/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "sized"
+version = "0.1.0"
+edition = "2021"

exercises/04_traits/08_sized/src/lib.rs

@@ -0,0 +1,7 @@
+pub fn example() {
+    // Trying to get the size of a str (or any other DST)
+    // via `std::mem::size_of` will result in a compile-time error.
+    //
+    // TODO: Comment out the following line and move on to the next exercise.
+    std::mem::size_of::<str>();
+}