Support arbitrary messaging from external loaders

[mattpolzin]

The idea with this change is to allow an ExternalLoader to pass messages back to whatever is actually kicking dereferencing off. These messages can look like anything that happens to be useful.

Breaking Changes

• The ExternalLoader protocol's load() function needs to return an array of "messages" now. If you don't need this parameter, you can set the type of the message to Void and always return an empty array. The new return is there to facilitate you passing information back to yourself from the load() function.