Re-arrange TOC.

I'm going to move testing to be right AFTER the guessing game. I wanted it
to be borderline TDD, but I think that, since the first example is just one
file, it might be a bit overkill.

I'm doing this in its own commit to hopefully avoid merge conflicts.

src/doc/guide.md

@@ -421,16 +421,6 @@ return
 
 comments
 
-## Testing
-
-attributes
-
-stability markers
-
-## Crates and Modules
-
-visibility
-
 ## Compound Data Types
 
 Tuples
@@ -451,10 +441,21 @@ loop
 
 break/continue
 
-iterators
+## iterators
 
 ## Lambdas
 
+## Testing
+
+attributes
+
+stability markers
+
+## Crates and Modules
+
+visibility
+
+
 ## Generics
 
 ## Traits