Make `traceDyn` possible to use with the eventlog

[mpickering]

I want to send the traceDyn traces to the eventlog using traceEvent rather than trace.

It would be easy to add some new functions traceEventDyn and traceEventDynWith which did this but this is confusing compared to traceEventWith and the existing Reflex.traceEvent.

I thought perhaps exposing a logDyn :: (a -> b -> b) -> Dynamic t a -> Dynamic t a function might be good. If someone can suggest a clean API where the names are not too confusing then I can implement it.

[mpickering]

Perhaps would also be good to be able to easily instrument calls to performEvent as they could block for a reasonable amount of time.