Inspect TODOs in source code; fix whatever we can

(This is a placeholder ticket, made so that people can find it when they search for things to do under the First Contribution label.)

In our source code, we often use the string TODO to indicate a place where we want to come back and improve things. You can sometimes find good starting projects by looking for this string in the code.

Unfortunately, some of these TODO issues are harder than they initially appear: therefore, it might be a good idea to ask about the intended fix before you start on one of them. Feel free to ask here, or on the #tor-dev IRC channel on OFTC.

Happy hacking!