Improve API for checking the signature in check_function_def

[filipsch]

If you look at the examples in the reference documentation of check_function_def(), you'll see that it's currently rather clunky to verify the signature of a function definition:

• A lot of work is needed to verify the number of arguments and whether or not defaults are set.
• You have to specify custom messages because pythonwhat doesn't automatically generate messages.

This should be improved. Not sure how.