make Vec<T> implement the slice methods

[thestinger]

It doesn't have auto-slicing, nor do I think it should have it, so this is necessary to make it usable without a lot of pain.

[darnuria]

The goal is to implement it like in python?With [start: end: step]? Or just [start:end]?

(Thats just a question I dont have the lvl for coding it).

Maybe this bug will need precisions like "easy or hard label" etc...

[huonw]

@darnuria that's something different and covered by #4160, @thestinger is referring to the methods like .head and .iter on &[T] (which is called a slice).

In any case, many of these methods are added in #12253.

[darnuria]

@thestinger Maybe this bug need to be closed? :)

[Gankra]

Impl'd through deref magic.