Reducer helpers for optimizing updates

[andywhite37]

From a discussion with @mlms13

It might be handy to add some helper functions for filtering actions in the reducer that would result in a no-op/unnecessary Update.

One example is if your state is an AsyncData or AsyncResult, and it's currently in an Init state, and you dispatch an action that wants to map a function over your Complete(Ok(a)) value, but you're not in that state, it's better to return a NoUpdate rather than an Update that has no effect.

Example:

Update(AsyncResult.map(v => {...v, someChange}))

vs.

getOk >> fold(NoUpdate, v => Update(...))

There might be some handy helper functions for dealing with this to avoid unnecessary updates - it would be worth thinking about.

[andywhite37]

As a side note, we should benchmark if there are any actual consequences of dispatching an Update with no state changes - it's possible react optimizes this away somehow, or it's insignificant.