Add a user exposed way of tracking performance

[jakemac53]

With lazy builds especially it is extremely difficult to track performance in a general way. I think we need to expose to the Builder implementors a way of explicitly tracking the things they care about.

I see at least three potential ways this could be exposed:

• Add a method to BuildStep.
• Add a top level method similar to log which uses an instance of a tracker hanging off the current zone.
• Create a new package which exposes a performance tracker as a Resource. It could log performance information in its dispose callback. This would have the benefit of separating the details out of the build package itself.

[natebosch]

This would have the benefit of separating the details out of the build package itself.

I definitely like the idea of being able to opt in to this separately from the rest of Build and not even be exposed to it otherwise.

[jakemac53]

This is done now