change event handlers to (event, datum)

[gordonwoodhull]

d3compat.eventHandler currently takes a function(datum, event) for trivial adaptation of old event handlers.

However, to reduce confusion and facilitate eventual removal of d3@5 compatibility, I think these should use the d3@6 signature.

Of course, we might not get to it until then; if not, this is documentation. It's something I noticed when working on #1824 when fixing tests to call consistently with the active version of d3. d3compat.callHandler takes a function(event, datum)