Ability to output data about write item creation times

[dsherret]

It would be very useful to get some data about when each underlying "write item" is created ("write items" are the final very simple IR that the printer builds up).

This wouldn't be done in release mode obviously.

The end data could then be used to make a visualization that shows where the printer is slow. Probably just a single html file output that shows the file text going from a blue text colour (fast) to red (slow).

[dsherret]

This is essentially #154. It's basically implemented now. Will do remaining work in #354.