feat: implement Body for empty tuple

[tottoto]

Implements Body for empty tuple, which is similar to Body for String.

[seanmonstar]

I've been hesitant about this implementation, since it means any function returning "nothing" is returning an impl Body. I think users probably would end up finding that more surprising than convenient. (I also think the String imply was perhaps a mistake, but can't take it back now...)

In a similar way, neither Future nor Stream are implemented for ().

[tottoto]

Makes sense. I agree that it is reasonable not to implement this.