memo-clear!

Reaches into an core.memo-memoized function and clears the cache. This is a destructive operation and should be used with care. When the second argument is a vector of input arguments, clears cache only for argument vector. Keep in mind that depending on what other threads or doing, an immediate call to `snapshot` may not yield an empty cache. That's cool though, we've learned to deal with that stuff in Clojure by now.