Given an object and a keypath, gets the value at that path. e.g. Get ['countries', 'US', 'states', 'MA', 'cities', 'Somerville'] in an addresses object.
A callback wrapper that will handle adding things passed to the callback to a 'channel'. A channel is an object that is being passed throughout a callback chain. Each callback may add things to that channel and look up things in that channel.