missing ergonomics to merge roles together

[nasser]

There isn't a an easy way to merge several defroles into a single roles map and I find myself writing repetitive code like:

(def bullet
  {:age-and-remove age-and-remove
   :tube-position tube-position
   :damage-things damage-things})

Some ergonomic functions to make the (possibly common) case where roles are singletons on their keys a little easier would be welcome here.