Document trait objects in the book

[krdln]

Trait objects seem not to be documented at all in the book, which together with their mandatory lifetime bound annotation makes them almost undiscoverable. Fortunately, there are some nice blog posts by @huonw, but they are unlikely for a user to stumble upon.

Also, the only occurence of 'trait object' in the reference is following sentence, which is strange:

Likewise, supertrait methods may also be called on trait objects.

[steveklabnik]

Yes, this has been on my list a long time.