Add new section on trait bounds.

exercises/04_traits/10_assoc_vs_generic/Cargo.toml

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+[package]
+name = "assoc_vs_generic"
+version = "0.1.0"
+edition = "2021"

exercises/04_traits/10_assoc_vs_generic/src/lib.rs

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+// TODO: Define a new trait, `Power`, that has a method `power` that raises `self`
+//  to the power of `n`.
+//  The trait definition and its implementations should be enough to get
+//  the tests to compile and pass.
+//
+// Recommendation: you may be tempted to write a generic implementation to handle
+// all cases at once. However, this is fairly complicated and requires the use of
+// additional crates (i.e. `num-traits`).
+// Even then, it might be preferable to use a simple macro instead to avoid
+// the complexity of a highly generic implementation. Check out the
+// "Little book of Rust macros" (https://veykril.github.io/tlborm/) if you're
+// interested in learning more about it.
+// You don't have to though: it's perfectly okay to write three separate
+// implementations manually. Venture further only if you're curious.
+
+#[cfg(test)]
+mod tests {
+    use super::Power;
+
+    #[test]
+    fn test_power_u16() {
+        let x: u32 = 2_u32.power(3u16);
+        assert_eq!(x, 8);
+    }
+
+    #[test]
+    fn test_power_u32() {
+        let x: u32 = 2_u32.power(3u32);
+        assert_eq!(x, 8);
+    }
+
+    #[test]
+    fn test_power_ref_u32() {
+        let x: u32 = 2_u32.power(&3u32);
+        assert_eq!(x, 8);
+    }
+}