Use slicing syntax instead of methods and add a feature gate

cc @aturon 

r? anyone?
I think it is a choice - when looking up the trait method we don't autoderef the receiver (self has type `&&[T]` here) - that is just a flag we pass in to the trait matching algorithm though, so it should be easy to change. However, I was following what index does, I assume we should be consistent with that, one way or the other.