Convert solve tracing system to operate over a channel

[sdboyer]

Right now, the solving tracer operates by writing directly to a log.Logger. This is suboptimal for implementors, as it means they can really do very little except pass the data through directly. It would be far better to instead optionally return a channel (probably from Prepare()) over which traceable events are sent, then abstract the current visualization into a standalone component that can consume those events and produce the same viz.

This would allow implementations to create their own visualizations, meta-analyses, or log the information in whatever other ways they see fit.

Unfortunately, this is may be blocked by #20, as the viz is quite tightly coupled with the traceError interface for now, and that's unlikely to evolve until we systematically attack errors.

[fabulous-gopher]

This issue was moved to golang/dep#432