changes should work on pulling behavior

[paldepind]

Hard to implement but would be nice.

Use case:

const isAfterEnd = timeB.map((t) => t >= duration);
const afterEnd = filter((b) => b, changes(isAfterEnd));

However, I think that implementing this would require Stream to also have a push/pull mode just like behaviors. I.e. the afterEnd stream above would be in pull-mode.