Return more chunks from Take::chunks_vectored

[vorner]

Specialize the Takes implementation of the method so it can return as many chunks as possible, instead of the default 1.

Implements #474.

(I'm not marking it „Closes“, as there's still the open question if the guarantee in the documentation should be relaxed; I think this makes sense even if it gets relaxed because it allows for more optimal use of write_vectored and similar).