on_update delta calculations are now lazy

[texodus]

As it says on the tin. This changes the default on_update API to invoke its callback function with no arguments. The previous behavior, which called the callback with the modified rows (but only worked correctly for 0-sided contexts, and performs poorly) can be preserved via

view.on_update(callback, {mode: "rows"});

Future updates will use the new mode optional parameter to implement a more granular update delta API, as well as improve the delta calculation performance penalty.