[RFC] Add allocators for zero-copy conversion from Box<T> into Rc<T>

[mahkoh]

The motivating example is

let mut vec = Vec::new_in(RcAlloc::new());
file.read_to_end(&mut vec)?;
let rc: Rc<[u8]> = vec.into_boxed_slice().into();

But note that read_to_end does not yet support allocators.