bbq2 does not support split reads

[Sympatron]

#77 added support for reading both parts of already committed data to allow for true zero copy operation. bbq2 seems to miss this feature as far as I can see.
Is this true or did I miss something?

[titaniumtraveler]

bbq2 is indeed missing the feature, but at least conceptually it should be relatively easy to re-add, as the holder of the read grant should theoretically be allowed to access the entirety of the committed buffer.

Though in practice it might be a bit more complicated, as adding support for it would require adding a method for this to Coord + implementing that method for all implementors.

[Sympatron]

I started working on this in #122. adding this to Coord and it's implementors was quite easy. It's basically the same as before.
What I having some trouble with is how to implement the SplitGrantR. I suppose it should be it's own type alongside StreamGrantR and FramedGrantR. But should it be returned from a split_read method on StreamConsumer or should I add a completely new SplitConsumer?
Also both other grants implement Deref<Target = [u8]> which would not be possible in this case. That would mean that I would have to provide a pub fn bufs(&self) -> (&[u8], &[u8]) method on SplitGrantR as before.
@jamesmunns do you agree?