The current implementation of Function Specialization does not allow specializing more than one arguments per function call, which is a limitation I am lifting with this patch.

My main challenge was to choose the most suitable ADT for storing the specializations. We need an associative container for binding all the actual arguments of a specialization to the function call. We also need a consistent iteration order across executions. Lastly we want to be able to sort the entries by Gain and reject the least profitable ones.

MapVector fits the bill but not quite; erasing elements is expensive and using stable_sort messes up the indices to the underlying vector. I am therefore using the underlying vector directly after calculating the Gain.