function ap (fs, xs) {
  return combine(apply, fs, xs)
}
Assume fs is a stream containing functions, and apply the latest function
in fs to the latest value in xs.
fs:         --f---------g--------h------>
xs:         -a-------b-------c-------d-->
ap(fs, xs): --fa-----fb-gb---gc--hc--hd->
@param {Stream} fs stream of functions to apply to the latest x
@param {Stream} xs stream of values to which to apply all the latest f
@returns {Stream} stream containing all the applications of fs to xs