Deref and DerefMut for smart pointer ergonomics

Implementing Deref lets your type automatically coerce to its target type. For example, Box<T> derefs to T, and String derefs to str. This enables method calls and conversions without explicit unwrapping. I implement Deref for wrapper types and smart pointers. DerefMut is the mutable version. The key is to only use deref coercion for pointer-like types, not as a general inheritance mechanism (Rust doesn't have inheritance). Deref coercion makes smart pointers feel natural: you can call methods on a Box<T> as if it were a T. It's a small feature that improves ergonomics significantly.