Function add_func 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn add_func(
    df: *mut Defuns,
    name: *const c_char,
    p: *const usize,
    num_args: usize,
)

Adds a new function to a Defun.

Safety

1, df should point to a valid Defun object created by create_defun.
2. name should be a valid utf8 string.
3. p should point to a valid C-styple function pointer that accepts
    num_args double arguments.