Use actions to target diffing instead of diffing the whole tree

Diffing should use the location of actions to more exactly target what it's diffing.  That was the initial idea, but I introduced a sort of ugly workaround in #68 because it wasn't doing that yet, so an effect handler with the same state at the top of the hierarchy would freeze the page, essentially.

This should clean up the tests for diffing that are sending extra, unneeded updates.