fix(async): flatten timeout result

Used to be that timeout would resolve into one of these cases:

• Ok(Ok(v))
• Ok(Err(e))
• Err(Expired)

Now the cases are:

• Ok(v)
• Err(Error::Failed(e))
• Err(Error::Expired)

As a side effect only futures that resolve into std::result::Result<T, E> can be wrapped with a Timeout future, which is the majority of cases.