Applicative instance does not separate concurrent events

[paf31]

From https://github.com/paf31/purescript-behaviors/issues/28#issue-277348392, if we have an event e, then e <*> e results in two events at each time point at which e fires, with the first of each pair reporting an "inconsistent" state.

Whether this is a bug or not is debatable, and I think to fix it would require some sweeping changes, but I want to keep track of it here.

[safareli]

@paf31 do you see any possible fix here?