'pi' notation for the projection operators, which are still not used outside tests
completed initial cut of NativeFiniteSets
completed NativeFiniteSets with tests
partial refactoring of bifunctionAsBiQuiver (can't do it all the way)
initial refactoring to get rid of EqualizerElement
further refactoring to get rid of EqualizerElement
completely got rid of EqualizerElement and EQUALIZER_ELEMENT
introduce concept of 'fletching' arrows