Add “More to Explore” sections (#1369)

From a discussion in #1313.

STYLE.md

@@ -65,6 +65,24 @@ collapsed or removed entirely from the slide.
   course, instructors will only have time to glance at the notes so it is not
   useful to include full paragraphs which the instructor should read out loud.
 
+### More to Explore
+
+Move extended explanations in the notes to a "More to Explore" section:
+
+```markdown
+<details>
+
+...
+
+## More to Explore
+
+...
+
+</details>
+```
+
+The material there is outside the scope of the regular class.
+
 ## Translations
 
 This section is about what you write in the translation. We describe

src/enums/sizes.md

@@ -58,6 +58,10 @@ Key Points:
      * `dbg_size!(&i32)`: size 8 bytes, align: 8 bytes (on a 64-bit machine),
      * `dbg_size!(Option<&i32>)`: size 8 bytes, align: 8 bytes (null pointer optimization, see below).
 
+## More to Explore
+
+Rust has several optimizations it can employ to make enums take up less space.
+
  * Niche optimization: Rust will merge unused bit patterns for the enum
    discriminant.
 

src/memory-management/stack.md

@@ -29,7 +29,10 @@ fn main() {
 
 * If students ask about it, you can mention that the underlying memory is heap allocated using the [System Allocator] and custom allocators can be implemented using the [Allocator API]
 
-* We can inspect the memory layout with `unsafe` code. However, you should point out that this is rightfully unsafe!
+## More to Explore
+
+We can inspect the memory layout with `unsafe` Rust. However, you should point
+out that this is rightfully unsafe!
 
 ```rust,editable
 fn main() {