Extend traits chapter

[japaric]

• supertraits FooBar: Foo + Bar
• trait objects Box<Foo>, &Foo
• "function overloading" using double dispatch: Vec * Vec and Vec * int

[Screwtapello]

Once you tell people that a struct can have implementations of any number of traits and the compiler will automatically look up which one to use, the immediate next question is "what happens when there's multiple traits with the same method?"

It would be helpful to have an example to point people to when they ask.

[steveklabnik]

Yup, all this would be great!

I'm happy to mentor anyone who wants to work on this issue.

[Morsicus]

I will be happy to help :)

[epage]

Another topic that would be useful for traits: comparing

1. generics (this might have impact on Consider moving generics/gen_trait (to traits?) #913).
2. boxed traits
3. trait references

For trait references, I forgot I could even do this and had been using generics until my C++ mindset kicked in and realized it should work and had to prove it to myself.