Returns a map containing all the keys of map1 and map2, where the
value for each key contained in only one map is the value from that map, and
the value for each key contained in both maps is the result of calling
val-fn on the value from map1 and the value from map2. val-fn
defaults to simply returning its second argument, so the entries in map2
simply shadow those in map1. The default for the new map is the result of
calling val-fn on the defaults for the two maps (so be sure it can take
these values).