Add a PeekableIter trait in standard library.

[PhilLavoie]

We already have a Peekable concrete type that makes it easy for already defined iterators to provide a peek function. I would like to suggest adding a standard trait that defines peekable iterator's behavior: peek and is_empty and next so that users can provide their own optimized peekable iterators, possibly avoiding the intermediate copy made by Peekable for example, but also because this:

fn parse< C, P: PeekableIter< C > >(iter: P) {...}

Looks better than

fn pase< C, I: Iterator< C > >(iter: Peekable< C, I >) {
  ...
 //Keep in mind that peekable returns a reference and not the actual C type.
 ...
}

[steveklabnik]

Additions to the standard library require an RFC these days. If you still want this, please persue one over there. Thanks!