Have a way to tell the tracer to completely skip tracing in some cases

[ionelmc]

Python allows tracers to:

• skip being called for line events (f_trace_lines=False for Python 3.7+)
• skip being called for subsequent calls (don't return the trace func or don't set f_trace for pure or C implementations)

Hunter has some filters that could easily benefit from those (currently Hunter's always drills down):

• depth
• depth_lt
• kind_in='return call exception'
• kind_nin='line' or kind_neq='line' (not implemented yet)

There could we two ways to tackle this:

• Implement some sort of LeanTracer that does filtering itself.
• Augment current tracer to have skip flags and checks in Query to set said flags. The trick would be how to detect that Query is root one (and only then perform the skips).

Ref #45.