BufWriter: apply #[inline] / #[inline(never)] optimizations
BufWriter: apply #[inline] / #[inline(never)] optimizations

Ensure that `write` and `write_all` can be inlined and that their
commonly executed fast paths can be as short as possible.

`write_vectored` would likely benefit from the same optimization, but I
omitted it because its implementation is more complex, and I don't have
a benchmark on hand to guide its optimization.

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