Haven't had a chance to look closely at the new version yet, but looks pretty good. Couple initial comments:
* Definite vote for std.experimental. Should get a bunch of folks to bang on it before the API has to be locked down. Having it as a dub package first has made it much easier to pick up and use, feel like this is a good path for all std packages to take - get it out there and iterate with user input before pulling in to std.
* The 'Tracer' doesn't feel like it belongs here, and if I understand things correctly won't actually work properly anyway, so should probably be removed.
* What others have said about string formatting. I poked around with this a bit before, but wasn't able to get a clean solution, should probably try again before complaining...