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Further minimalist version of https://github.com/mattjj/autodidact.

Ordered Todo:

Implement tracer.backwardpass for scalar-valued function taking gradient w.r.t one variable
Implement tracer.backwardpass for arbitrary graph (given forward leaf values, compute backward leaf values)
Wrap basic numpy functions

Time independent Todo:

Implement isBox via type mappings intead of via python builtin isinstance for a speedup