performance api

[mhuebert]

Example of using the performance api to measure event-handling timings. These can be nicely viewed in devtools Performance tab profiles. In the example below, we see the doc/tx event taking the majority of time spent in the keypress event (these timings are shown in the upper section, while the call stack is shown at the bottom):

This PR is not production-ready - that would require an ability to turn this on/off, and ensure that the apis are available in the browser before using them, as well as further thought as to what exactly we should be measuring.

[plexus]

Cool stuff!