Moved some of the code of PolyHashTable into its implementation file.
Clean up of Reducer header file and remove statistics functionality from there. If we need that functionality in future, it should be implemented using the logging facility, not as a part of the main-line code.
Made registering a reducer type depend on a decentralized method instead of having a big function in Reducer.cpp do it, leading to long compile times when that file needs to be recompiled. The new method is now implemented and being used for ReducerPack. The remaining reducers are using the new method, but it is still being done from Reducer.cpp. Next up: migrate all new reducers to this new way of doing it.