Export the names of the functions used in each element

See #79

This is a proof of concept for exporting the functions used by each element. I've made no attempt to import this data here, because we haven't decided the best way of doing that yet. The idea is to maybe go ahead and get this committed so folks can hack away on the importing/display part.

Notes:
- This only covers calls to regular functions. Class methods are different and can be added later.
- I chose to export `do_action` and `apply_filter` calls, even though they are already being exported as hooks. My thinking here is that they will be weeded out on import or display as needed.