Pull / Lazy signals support

[sirinath]

There are times when on value updates we do not want to compute everything unless there is consumer for the value.

So is it possible to add push / pull model. E.g.

x = Signal(0)
push!(x, 1)

y = pull(x)
z = foldp(+, 0, y)
value(z)

z is updated only when needed. Assume z is a very expensive calculation.

Also,

x = mapLazy(a -> a + 1)
value(x)

and

x = foldLazy(+, 0)
value(x)

[yuehhua]

+1
I think the pull / push model is the basic of reactive programming.
To achieve this, wrapping operations into a Task (or coroutine) is necessary.