No trailing spaces. Code style is more consistent
Add LEN function. Clean up the code a bit.

--Make it easier to add function names to the str array and have the
  additional length propagate to the other stuff that depends on it in
  the future.
--Make MUL and ADD handle a NULL returned by evaluateList
--Forgot that + doesn't work on strings in C. Apparently I didn't need
  anything for it to automatically concatenate the error message split
  across two lines.
--Add example to test the LEN function.