BufWriter: use #[cold] and less aggressive #[inline] hints
BufWriter: use #[cold] and less aggressive #[inline] hints

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