Two package:unordered-containers

Create a map from two key-value pairs which hashes don't collide. To enhance sharing, the second key-value pair is represented by the hash of its key and a singleton HashMap pairing its key with its value. Note: to avoid silly thunks, this function must be strict in the key. See issue #232. We don't need to force the HashMap argument because it's already in WHNF (having just been matched) and we just put it directly in an array.