Generalize chunks/ready_chunks/try_chunks/etc to collect into T: Extend, not just Vec

[khuey]

In particular I would find it really useful to be able to ready_chunks a Stream of Futures into FuturesOrdereds and spawn them on a threadpool to parallelize my computation.

This would be a breaking change of course.

[taiki-e]

Seems reasonable to me.

[khuey]

This is actually a bit harder than I thought. Ideally there would be traits for with_capacity and len too so that we can reserve space in the collection and not have to track the collection's size again ourselves in Chunks.

[ibraheemdev]

There is extend_reserve on nightly.

[khuey]

Yeah people don't seem eager to stabilize it though, and there's nothing at all for len.