Update the TOC for ownership.

Here's a rough summary of notes from @aturon and I's discussion about
this:

Topics:
    - heap vs stack
    - pointers
    - slices
    - scope/drops

- Ownership
    - confusion around allocating strings vs literals
    - String::new and String.push
    - or do we use String::from?
    - scope/drops
     - Move vs copy

- Borrowing
    - Rust's take on pointers to entire objects

- Memory layout
    - heap vs stack -- what does it mean to own a String -- what even is
      a String??
    - talk in terms of box and arrow diagrams, rather than address
      spaces etc
    - keep a high level of conceptual abstraction
    - pointers

- Slices
    - Rust's take on *interior* pointers
    - Niko argues that slices and borrowing from fields of a struct are
      conceptually related
    - Sing the praises of slices wrt safety!
    - We get to address string literals very early!
    - Sidebar on literals and rodata

- Advanced: lifetimes

src/SUMMARY.md

@@ -16,10 +16,12 @@
     - [Control flow with `if`](if.md)
     - [Loops](loops.md)
 
-- [Ownership & borrowing]()
+- [Understanding Ownership](understanding-ownership.md)
     - [Ownership]()
     - [References & Borrowing]()
-    - [Lifetimes]()
+    - [Memory Layout]()
+    - [Slices]()
+    - [Advanced: Lifetimes]()
 
 - [Structs](structs.md)
     - [Method Syntax]()

src/understanding-ownership.md

@@ -0,0 +1,6 @@
+# Understanding Ownership
+
+Now that we’ve got some basic syntax under our belt, it’s time to take a look
+at Rust’s most unique feature: ownership. We’ll also talk about several related
+features: borrowing, slices, and lifetimes, as well as how Rust lays things out
+in memory.