Make Hyper v0 support optional

[PaulDance]

Dear maintainers,

Now that Hyper v1 has been supported for a few versions and that it works fine, it seems like a good time to consider retiring v0. Currently, only adding v1 is possible without forking, thus doubling the hyper stack's dependencies instead of replacing them, since v0 is always enabled even though unused.

Would it therefore be possible to either remove v0 or at least make it optional? Making the v0 module get behind cfg(not(feature = "hyper1")) seems to work. The idea would be to have the full re-export kept for both modules so that only the Cargo feature controls what is selected and passing from one to the other would be a drop-in, at least from the item paths point of view. Not pulling any v0 when v1 is enabled and vice-versa would be a priority.

I've made an attempt at this in #304.

Thanks in advance,
Paul.