Add traits for concat and connect methods

[brendanzab]

This adds the following traits to prelude:

In std::str, impled on &[~[T]] and &[&[T]]:

pub trait StrVector {
    pub fn concat(&self) -> ~str;
    pub fn connect(&self, sep: &str) -> ~str;
}

In std::vec, impled on &[~str] and &[&str]:

pub trait VectorVector<T> {
    pub fn concat(&self) -> ~[T];
    pub fn connect(&self, sep: &T) -> ~[T];
}