Convert an ObjectId to a hex string in aggregation pipeline stages

At the moment if you need to concatenate two ObjectIds into a single string value then this has to be done outside AP (in Java code, etc).

What would be really handy is if there was so sort of $toString function that converts anything to a String representation or if you wanted to cut down the scope for now then expand the $concat function to internally convert ObjectIds to their hex string representation during the concat operation.