Resolves #4101.

combineWith takes a function to specify how to merge the intervals of two schedules, but it is always done if either schedule is done so to a certain extent it already embodies intersection semantics in that one schedule can no longer continue if the other one doesn't want to.

This PR renames combineWith to unionWith and implements a new combinator intersectWith that still takes a merge function but now continues as long as either schedule wants to continue. || can then be implements in terms of unionWith to make use cases like Schedule.recurWhile[Boolean](_ == true) || Schedule.fixed(1.second) work.