Add an extra opaque parameter for the c_function to store some data.

This is useful if you have to funnel a bunch of c_functions through a common
callback function (when writing a language adapter, for instance).
[eval_apply.cpp] Set c_args.list.length when calling c_functions.
Merge pull request #118 from caldwell/add_cookie_to_c_function

Add an extra opaque parameter for the c_function to store some data.