Auto merge of rust-lang#25009 - alexcrichton:less-buffered-stream, r=…

…aturon

As pointed out in rust-lang#17136 the semantics of a `BufStream` aren't always what one
expects, and it looks like other [languages like C#][c-sharp] implement a
buffered stream with only one underlying buffer. For now this commit
destabilizes the primitive in the `std::io` module to give us some more time in
figuring out what to do with it.

[c-sharp]: https://msdn.microsoft.com/en-us/library/system.io.bufferedstream%28v=vs.110%29.aspx

[breaking-change]

1.0.0-alpha.2 release

1.0.0-alpha release

0.12.0 release

0.11.0 release

0.10 release

0.9 release

0.8 release

0.7 release