rust: Stream ownership is cumbersome

[alexcrichton]

Currently streams implement Sink and Stream for Vec<T>, but this has a number of implications:

• For primitives such as stream<u8> that means that sending chunks of data requires sending chunks of allocations. There's no chance to reuse buffers or otherwise have more efficient buffer handling.
• For non-primitives such as stream<list<u8>> it's possible that a write doesn't fully send everything. There's no way to recover items which weren't sent in a chunk. I realize right now that write-as-generated writes everything in a loop, but I think this is probably also something worth changing.
• If a write is cancelled there's no way currently to recover the original chunk of items that were being sent.