Make checker trace more granular

[ranjitjhala]

          I think we want more granularity than this because this is also going to dump the trace for the shape phase, which I don't think we ever want to expose to users. Maybe we can make `config::dump_checker_trace` return an enum with three levels (none, only check phase, all). WE would also need to pass the phase to this macro. 

We can implement this later though.

Originally posted by @nilehmann in #915 (comment)