Auto merge of #25009 - alexcrichton:less-buffered-stream, r=aturon
Auto merge of #25009 - alexcrichton:less-buffered-stream, r=aturon

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]

jroesch/rust,nwin/rust,jroesch/rust,l0kod/rust,mdinger/rust,victorvde/rust,aneeshusa/rust,omasanori/rust,reem/rust,gifnksm/rust,aepsil0n/rust,sae-bom/rust,GBGamer/rust,rohitjoshi/rust,TheNeikos/rust,nwin/rust,aidancully/rust,zubron/rust,kwantam/rust,aepsil0n/rust,victorvde/rust,ejjeong/rust,mdinger/rust,KokaKiwi/rust,KokaKiwi/rust,dwillmer/rust,cllns/rust,vhbit/rust,nwin/rust,zubron/rust,sae-bom/rust,GBGamer/rust,graydon/rust,aidancully/rust,l0kod/rust,dwillmer/rust,l0kod/rust,l0kod/rust,pshc/rust,jashank/rust,gifnksm/rust,nwin/rust,mvdnes/rust,aidancully/rust,andars/rust,vhbit/rust,graydon/rust,mihneadb/rust,mihneadb/rust,aepsil0n/rust,GBGamer/rust,omasanori/rust,carols10cents/rust,vhbit/rust,cllns/rust,carols10cents/rust,mdinger/rust,ejjeong/rust,GBGamer/rust,sae-bom/rust,graydon/rust,graydon/rust,pelmers/rust,gifnksm/rust,seanrivera/rust,TheNeikos/rust,aepsil0n/rust,zachwick/rust,jroesch/rust,dwillmer/rust,l0kod/rust,vhbit/rust,carols10cents/rust,pshc/rust,philyoon/rust,reem/rust,zubron/rust,robertg/rust,graydon/rust,pshc/rust,dwillmer/rust,reem/rust,dwillmer/rust,carols10cents/rust,pelmers/rust,jashank/rust,aneeshusa/rust,krzysz00/rust,zachwick/rust,aneeshusa/rust,kwantam/rust,l0kod/rust,jroesch/rust,mihneadb/rust,robertg/rust,GBGamer/rust,rohitjoshi/rust,KokaKiwi/rust,rohitjoshi/rust,robertg/rust,vhbit/rust,GBGamer/rust,jroesch/rust,philyoon/rust,kwantam/rust,mvdnes/rust,kwantam/rust,cllns/rust,mvdnes/rust,mdinger/rust,jashank/rust,zachwick/rust,XMPPwocky/rust,kwantam/rust,mdinger/rust,krzysz00/rust,nwin/rust,pshc/rust,rohitjoshi/rust,rohitjoshi/rust,sae-bom/rust,omasanori/rust,vhbit/rust,seanrivera/rust,dwillmer/rust,kwantam/rust,pelmers/rust,TheNeikos/rust,XMPPwocky/rust,aepsil0n/rust,jashank/rust,philyoon/rust,krzysz00/rust,mvdnes/rust,gifnksm/rust,TheNeikos/rust,ejjeong/rust,philyoon/rust,mdinger/rust,philyoon/rust,l0kod/rust,jashank/rust,victorvde/rust,zubron/rust,aidancully/rust,jashank/rust,zachwick/rust,seanrivera/rust,krzysz00/rust,andars/rust,omasanori/rust,pelmers/rust,pshc/rust,seanrivera/rust,pshc/rust,dwillmer/rust,rohitjoshi/rust,XMPPwocky/rust,GBGamer/rust,cllns/rust,andars/rust,jroesch/rust,carols10cents/rust,KokaKiwi/rust,pelmers/rust,ejjeong/rust,gifnksm/rust,mihneadb/rust,cllns/rust,aneeshusa/rust,zubron/rust,andars/rust,mihneadb/rust,seanrivera/rust,TheNeikos/rust,aidancully/rust,andars/rust,aneeshusa/rust,victorvde/rust,GBGamer/rust,mvdnes/rust,omasanori/rust,pshc/rust,dwillmer/rust,victorvde/rust,zubron/rust,XMPPwocky/rust,seanrivera/rust,robertg/rust,reem/rust,zubron/rust,nwin/rust,reem/rust,sae-bom/rust,zachwick/rust,KokaKiwi/rust,ejjeong/rust,philyoon/rust,sae-bom/rust,krzysz00/rust,zubron/rust,mvdnes/rust,XMPPwocky/rust,omasanori/rust,andars/rust,zachwick/rust,XMPPwocky/rust,l0kod/rust,victorvde/rust,vhbit/rust,reem/rust,gifnksm/rust,aneeshusa/rust,pelmers/rust,jroesch/rust,aidancully/rust,carols10cents/rust,ejjeong/rust,graydon/rust,mihneadb/rust,vhbit/rust,jashank/rust,robertg/rust,krzysz00/rust,jroesch/rust,TheNeikos/rust,jashank/rust,nwin/rust,KokaKiwi/rust,robertg/rust,aepsil0n/rust,nwin/rust,cllns/rust,pshc/rust