Reduce references to niche optimization (google#2385)

Niche optimization is currently mentioned in three places:
 - Enums (User-Defined Types, Day 1 Afternoon)
 - Option (Standard Library Types, Day 2 Afternoon)
 - Box (Smart Pointers, Day 3 Morning)

This is a tricky thing to get right, and it was just in the speaker
notes in each place. google#1820 will introduce a fuller explanation.

Fixes google#1820.

src/smart-pointers/box.md

@@ -32,7 +32,7 @@ fn main() {
 from `T` directly on a `Box<T>`](https://doc.rust-lang.org/std/ops/trait.Deref.html#more-on-deref-coercion).
 
 Recursive data types or data types with dynamic sizes cannot be stored inline
-without a pointer indirection, which can be worked around using `Box`:
+without a pointer indirection. `Box` accomplishes that indirection:
 
 ```rust,editable
 #[derive(Debug)]
@@ -86,36 +86,8 @@ fn main() {
   have to use indirection, a `Box` or reference of some kind, instead of storing
   the value directly.
 
-# More to Explore
-
-## Niche Optimization
-
-Though `Box` looks like `std::unique_ptr` in C++, it cannot be empty/null. This
-makes `Box` one of the types that allow the compiler to optimize storage of some
-enums.
-
-For example, `Option<Box<T>>` has the same size, as just `Box<T>`, because
-compiler uses NULL-value to discriminate variants instead of using explicit tag
-(["Null Pointer Optimization"](https://doc.rust-lang.org/std/option/#representation)):
-
-```rust,editable
-use std::mem::size_of_val;
-
-struct Item(String);
-
-fn main() {
-    let just_box: Box<Item> = Box::new(Item("Just box".into()));
-    let optional_box: Option<Box<Item>> =
-        Some(Box::new(Item("Optional box".into())));
-    let none: Option<Box<Item>> = None;
-
-    assert_eq!(size_of_val(&just_box), size_of_val(&optional_box));
-    assert_eq!(size_of_val(&just_box), size_of_val(&none));
-
-    println!("Size of just_box: {}", size_of_val(&just_box));
-    println!("Size of optional_box: {}", size_of_val(&optional_box));
-    println!("Size of none: {}", size_of_val(&none));
-}
-```
+- Though `Box` looks like `std::unique_ptr` in C++, it cannot be empty/null.
+  This makes `Box` one of the types that allow the compiler to optimize storage
+  of some enums (the "niche optimization").
 
 </details>

src/std-types/option.md

@@ -30,7 +30,11 @@ fn main() {
     None.
   - It's common to `unwrap`/`expect` all over the place when hacking something
     together, but production code typically handles `None` in a nicer fashion.
-- The niche optimization means that `Option<T>` often has the same size in
-  memory as `T`.
+
+- The "niche optimization" means that `Option<T>` often has the same size in
+  memory as `T`, if there is some representation that is not a valid value of T.
+  For example, a reference cannot be NULL, so `Option<&T>` automatically uses
+  NULL to represent the `None` variant, and thus can be stored in the same
+  memory as `&T`.
 
 </details>