As pointed out in #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]
db477eef72