Refactor: use code/ dirs and lstinputlisting{}

[chrissprague]

To be more specific, each exam should have a code/ dir.

All "fully-fledged" (stand alone, etc.) src files should be included in that directory.

Instead of doing \lstlisting and putting the code itself into the questions, we can cut down the clutter significantly by keeping the programs as separate files, and use \lstinputlisting{code/[file]}.

An added benefit is that github will recognize that our exam covers several different languages.