rewrite part of the tutorial

This begins a rewrite of some sections the tutorial as an introduction
to concepts through the implementation of a simple data structure. I
think this would be a good way to introduce references, traits and many
other concepts too. For example, the section introducing alternatives to
ownership can demonstrate a persistent list.
auto merge of #10690 : thestinger/rust/doc, r=alexcrichton

This begins a rewrite of some sections the tutorial as an introduction
to concepts through the implementation of a simple data structure. I
think this would be a good way to introduce references, generics, traits
and many other concepts too. For example, the section introducing
alternatives to ownership can demonstrate a persistent list.