Unnecessary recalculation?

Hi, I'm evaluating several FRP solutions for a large-scale project and have been diving into the source code for this library. First of all, kudos for a great library, it's been working great with the prototypes I've been playing with and I'm greatly enjoying using it.

There's one point I don't understand - in the Elm implementation of FRP, signals pass an 'updated' flag through the signal graph to avoid unnecessary recalculation (see from page 28 in [Czaplicki's original paper](https://www.seas.harvard.edu/sites/default/files/files/archived/Czaplicki.pdf), and the [original implementation](https://github.com/elm-lang/core/blob/2.1.0/src/Native/Signal.js) of Elm's signals).

However, I can't seem to find this mechanism in the purescript-signal code - the entire signal graph seems to be updated for every input. Is there a reason for this, and would you accept a pull request to add it to the library?