Add Pipeline::apply()

[trowski]

I had the idea to expose a way for a mapper function to stop pipeline consumption early, like we do internally with take() and takeWhile(). This new apply() function plus Emission class was the result. It is then used to implement other methods such as take(), etc.

I'm not certain this is necessary. takeWhile() can fill this use-case, but that path does require defining the mapping and stop logic in separate functions.