Pyccolo

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* Remove bare except;
* Be robust to undefined builtins;

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* Hoist global / nonlocal declarations above instrumentation branch;

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* Add decorator trace event;

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* Skip pyccolo frames in pdb;

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* Improve ergonomics of trace stack get_field() with height param;
* Add option to disable tracing for non-main threads;
* Add is_initial_frame_stmt utility;

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* Fix a bug in the optional chainer example;