Small fixes for tutorial.

5.2 Simplify example to remove hidden #[deriving(Show)].
    Add example for constructing the enums.

8   Reference later sections describing rc, gc and send.
    Fix for #15293.

13  Simplify BananaMania example to remove hidden code.

17  Gave an example using the derived Rand trait.

Removed references to removed 'extra' crate.
auto merge of #15745 : treeman/rust/tutorial-fixup, r=steveklabnik

Simplify example in 5.2 to remove hidden `#[deriving(Show)]`. Traits haven't been introduced yet and now it's easier to just type in the code and expect it to work. Add in some examples for constructing the enum types. Explicitly expose `#![feature(struct_variant)]` in the code to make it more transparent, this bit me when I worked through the tutorial.

Add references in chapter 8 to later chapters describing `Rc`, `Gc` and `Send`. This is a simple fix for #15293.

Simplify vector indexing example in chapter 13 and removed hidden, unnecessary, code. Gave an example usage of the derived `Rand` trait in chapter 17.

Removed references to removed 'extra' crate.