gi: simplify gjs_invoke_c_function() argument bookkeeping

Make the out_arg_cvalues and inout_original_arg_cvalues arrays be the
same length as in_arg_cvalues, so that we don't need to keep a
separate counter for each. Rename the counter and length variables to
make it clearer what each one is counting.

https://bugzilla.gnome.org/show_bug.cgi?id=651558