Add Signal#getAndDiscreteUpdates

[armanbilge]

The motivation for this comes from Calico, where we want to get the current value of a Signal for immediate rendering, and also a handle to a Stream of discrete updates, that we can subscribe to at some later point (typically after rendering) (or not at all), without missing any updates.

FWIW I did try the "hack" of get followed by discrete.drop(1), and this ended up breaking in some situations.

I also tried inlining the default implementation here in Calico, but there is a noticeable performance difference (where by "noticeable", I mean visible UX latency in a webapp).

Suffice to say, the win comes from the fact that each Signal can override this with something more efficient (basically just a .get) than the default uncons1 implementation.

Furthermore, get and discrete can both be derived from getAndDiscreteUpdates much more easily than the other way around.

[mpilquist]

Need conflict mixup