Enable transducers to report to Racket Feature Profiler

[samdphillips]

Related to #351, but this is more about users writing and profiling their code.

Users should be able to use the feature profiler to figure out:

• how much time is being spent in transducer/reducer machinery (transduce, in-transduced, etc)
• how much time is being used per transducer in a pipeline

[jackfirth]

This is a very good idea. But I've never implemented a feature plugin before, or even used the feature profiler. Example code of other libraries doing this would be very helpful.

[samdphillips]

For reference:

• Part three of the Feature Profiler documentation is for library developers.
• PLT paper about the design/implementation of feature profiling

I haven't done this either. Browsing the core Racket parts it appears they use syntax properties a fair amount so it doesn't interfere too much with regular usage.

Still more reading and research to do. I just wanted to have this issue around so I wouldn't forget about it.