Rc/Arc are missing `try_new_uninit_slice_in`

[CieriA]

Hi,

Following issue #130, Box<T, A> now has the methods try_new_uninit_slice_in and try_new_zeroed_slice_in, alongside with all the other constructors.

However Rc<T, A> and Arc<T, A> miss try_new_uninit_slice_in and try_new_zeroed_slice_in, even if they have every other constructor Box<T, A> has.

Should they be implemented?