Add BufferOne sink combinator

[carllerche]

The decorator can buffer a single value without requiring any internal allocations. The type also provides poll_ready. I left out a Sink::buffer_one for this one in favor of BufferOne::new.

Relates to #409

[alexcrichton]

I wonder if we could perhaps push on the buffered type first before this? I think the semantics of poll_ready are implementable over there, right? We may also be able to special case w/ a "small vec" sort of

[carllerche]

Buffered could possibly be updated to handle this case. I agree it is worth a look.