Add discussion points for traits (#180)

* Some discussion points for traits
* Add answers

src/traits/drop.md

@@ -28,3 +28,15 @@ fn main() {
     println!("Exiting main");
 }
 ```
+
+<details>
+
+Discussion points:
+
+* Why does not `Drop::drop` take `self`?
+    * Short-answer: If it did, `std::mem::drop` would be called at the end of
+        the block, resulting in another call to `Drop::drop`, and a stack
+        overflow!
+* Try replacing `drop(a)` with `a.drop()`.
+
+</details>

src/traits/operators.md

@@ -20,3 +20,19 @@ fn main() {
     println!("{:?} + {:?} = {:?}", p1, p2, p1 + p2);
 }
 ```
+
+<details>
+
+Discussion points:
+
+* You could implement `Add` for `&Point`. In which situations is that useful? 
+    * Answer: `Add:add` consumes `self`. If type `T` for which you are
+        overloading the operator is not `Copy`, you should consider overloading
+        the operator for `&T` as well. This avoids unnecessary cloning on the
+        call site.
+* Why is `Output` an associated type? Could it be made a type parameter?
+    * Short answer: Type parameters are controlled by the caller, but
+        associated types (like `Output`) are controlled by the implementor of a
+        trait.
+
+</details>