Add new section on trait bounds.

exercises/04_traits/09_from/Cargo.toml

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

exercises/04_traits/09_from/src/lib.rs

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+// TODO: Implement the `From` trait for the `WrappingU32` type to make `example` compile.
+
+pub struct WrappingU32 {
+    value: u32,
+}
+
+fn example() {
+    let wrapping: WrappingU32 = 42.into();
+    let wrapping = WrappingU32::from(42);
+}