When API is stable, compare performance of ast vs. exec

[ericvsmith]

The tag last-version-with-ast points to a version of the code that uses ast to create functions. The current master branch uses exec.

[hynek]

JFTR, the reason why attrs went with exec is so it's possible to use pdb to step through our generated __init__.

[ericvsmith]

That's helpful, @hynek. Thanks for the insight.