Reimplement -switchToLatest using existing operators

From this [stackoverflow question](http://stackoverflow.com/questions/18291805/scanning-over-a-merge-of-differently-typed-signals), I was helped to realize that `-switchToLatest` could be implemented using `-flattenMap:` and `-takeUntil:`. This makes its implementation more succinct, and I reason the semantics are the same, but I haven't considered performance. I thought it was worth bringing it up.