Add the final code for the sixth example

This change-set adds the final code for the sixth example from the
"Learn Rust With Entirely Too Many Linked Lists" book.

Author: irbull

.github/workflows/main.yml

@@ -3,8 +3,8 @@ name: Rust
 on:
   push:
     branches:
-      - 'master'
-      - '!gh-pages'
+      - "master"
+      - "!gh-pages"
   pull_request:
 
 jobs:

lists/src/lib.rs

@@ -3,6 +3,7 @@ pub mod second;
 pub mod third;
 pub mod fourth;
 pub mod fifth;
+pub mod sixth;
 
 pub mod silly1;
-pub mod silly2;
+pub mod silly2;