core::at_vec should have from_owned and from_slice functions

[Dretch]

Currently core::at_vec has constructor functions including build, build_sized, build_sized_opt, from_fn and from_elem.

As far as I understand, none of these allow constructing an @vec by moving (rather than copying) all the values from a ~vec.

Therefore, I think it would be good if core::at_vec gained a function like:

pub fn from_owned<T>(v: ~[T]) -> @[T] { ... }

and, for convenience, something like this too:

pub fn from_slice<T:Copy>(v: &[T]) -> @[T] { ... }

Is this proposal something desirable? If so I will try and submit code/docs/tests.

[catamorphism]

I merged @Dretch 's change in 519b60f