Identify functions most in need of testing, and hardest to test

In addition to our opportunistic write-tests-as-you-go approach, we should also direct our testing efforts on the functions that most need to be tested. This includes (possibly)

• Big functions
• Volatile functions (ones that have changed many times)
• Functions that have had lots of bugs in them in the past
• Functions nobody understands
• Functions with high complexity
• Functions with high complexity-to-line ratio
• Functions with high code-to-comments ratio

We should also look at what's hardest to test, including possibly

• Anything that hits the OS directly
• Anything that's windows only.
• Anything that calls many other functions
• Anything that calls many other modules
• Anything that indirectly calls most of the rest of Tor.