tutorial: alternatives to ownership

This replaces the old section on managed pointers because the syntax is
going to be removed and it's currently feature gated so the examples
don't work out-of-the-box. Dynamic mutability coverage can be added
after the `Mut<T>` work has landed.
auto merge of #10589 : thestinger/rust/doc, r=pcwalton



This replaces the old section on managed pointers because the syntax is
going to be removed and it's currently feature gated so the examples
don't work out-of-the-box. Dynamic mutability coverage can be added
after the `Mut<T>` work has landed.