implement simple coverage functionality underneath tracer

[smacke]

We don't currently generate coverage reports because existing tools require tracing, which conflicts with our use of tracing. One option is to implement our own coverage functionality. Another option could be to somehow compose the tracing function used in other coverage tools with our own, and provide to sys.settrace(...).

[smacke]

this is done

[smacke]

Actually scratch that, it's not 100% done -- the coverage functionality has false negatives for stuff that gets executed under sys_tracer; possibly no good way around that but will reopen this issue to track.