Added standalone clone() and deep_clone() functions.

Right now, if you have an `Iterator<&T>`, like you get with `vec.iter()`, then for getting an `Iterator<T>` you have to do either of these:

~~~
// Only works for implicit copyable T, and can be confusing due to many pointers/sigils:
[1, 2, 3, 4].iter().map(|x| *x)...  
[1, 2, 3, 4].iter().map(|&x| x)...  

// Works for anything cloneable, but is long and can confuse the matters more due to autoderef:
[1, 2, 3, 4].iter().map(|x| x.clone())... 
~~~

These issues come regularly up while explaining `Iterators` to people on IRC, or for writing short understandable examples.

Having a standalone function of `clone` eases the situation: It's signature is `&T -> T`, and `map` transforms `A -> B`. Therefore `map(clone)` is easily explainable as removing an layer of `&` by making an copy through it:
~~~
[1, 2, 3, 4].iter().map(clone)... 
~~~

Just like `std::from_str::from_str`, these two functions are basically just workarounds for not being able to import associated functions from traits.