Rollup merge of #61235 - lzutao:stabilize-bufreader_buffer, r=Centril
Rollup merge of #61235 - lzutao:stabilize-bufreader_buffer, r=Centril

Stabilize bufreader_buffer feature

FCP done in https://github.com/rust-lang/rust/issues/45323#issuecomment-495937047

Closes #45323

r? @SimonSapin

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust