Add io write by_ref

[yjhmelody]

#131
Maybe it's a break change, because WriteExt and ReadExt both have by_ref.

[yoshuawuyts]

I tried doing this as part of #314 a while ago. I think the right way to go about this is to add the methods to the futures-rs implementations so they can be directly targeted.

I think this could indeed be considered a breaking change if we merged it here directly. My guess is we wouldn't break too much, but there's no harm in leaning a bit more on the conservative side here.