std: add a Writer impl for &mut [u8]

@alexcrichton mentioned in the [io reform rfc](https://github.com/rust-lang/rfcs/pull/517#issuecomment-66796223) that there might be interest in reviving the Writer for `&mut [u8]`, so I revived the PR for consideration. As written, it needs to do some unsafe magic in order to advance the mutable slice. It would be nice if this was someday factored out into a method on the slice itself.