io: replace Result<T, io::Error> with io::Result<T> in AsyncWrite

[paolobarbolini]

Motivation

std::io::Read, std::io::Write and std::io::Seek use the std::io::Result<T> type. So does tokio::io::AsyncRead. tokio::io::AsyncWrite instead follows the manual approach of std::result::Result<T, io::Error>.

This is very annoying because when implementing AsyncWrite with the help of rust-analyzer, this inconsistency is then propagated to the user's code.

Solution

This changes AsyncWrite to use std::io::Result.