auto merge of #19431 : erickt/rust/buf-writer-error, r=alexcrichton

Previously, `BufWriter::write` would just return an `std::io::OtherIoError` if someone attempted to write past the end of the wrapped buffer. This pull request changes the error to support partial writes and return a `std::io::ShortWrite`, or an `io::io::EndOfFile` if it's been fully exhausted.

 I've also optimized away a bounds check inside `BufWriter::write`, which should help shave off some nanoseconds in an inner loops.

diff --cc src/libstd/io/mem.rs
Simple merge